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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>16.4. argparse — Parser for command-line options, arguments and sub-commands — Python 3.6.7 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.6.7 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="16.5. getopt — C-style parser for command line options" href="getopt.html" /> <link rel="prev" title="16.3. time — Time access and conversions" href="time.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/library/argparse.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="getopt.html" title="16.5. getopt — C-style parser for command line options" accesskey="N">next</a> |</li> <li class="right" > <a href="time.html" title="16.3. time — Time access and conversions" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="allos.html" accesskey="U">16. Generic Operating System Services</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="module-argparse"> <span id="argparse-parser-for-command-line-options-arguments-and-sub-commands"></span><h1>16.4. <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> — Parser for command-line options, arguments and sub-commands<a class="headerlink" href="#module-argparse" title="Permalink to this headline">¶</a></h1> <div class="versionadded"> <p><span class="versionmodified">New in version 3.2.</span></p> </div> <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/argparse.py">Lib/argparse.py</a></p> <hr class="docutils" /> <div class="sidebar"> <p class="first sidebar-title">Tutorial</p> <p class="last">This page contains the API reference information. For a more gentle introduction to Python command-line parsing, have a look at the <a class="reference internal" href="../howto/argparse.html#id1"><span class="std std-ref">argparse tutorial</span></a>.</p> </div> <p>The <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module makes it easy to write user-friendly command-line interfaces. The program defines what arguments it requires, and <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> will figure out how to parse those out of <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a>. The <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module also automatically generates help and usage messages and issues errors when users give the program invalid arguments.</p> <div class="section" id="example"> <h2>16.4.1. Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2> <p>The following code is a Python program that takes a list of integers and produces either the sum or the max:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">description</span><span class="o">=</span><span class="s1">'Process some integers.'</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'integers'</span><span class="p">,</span> <span class="n">metavar</span><span class="o">=</span><span class="s1">'N'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'an integer for the accumulator'</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--sum'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'accumulate'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_const'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="nb">sum</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="nb">max</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'sum the integers (default: find the max)'</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="nb">print</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">accumulate</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">integers</span><span class="p">))</span> </pre></div> </div> <p>Assuming the Python code above is saved into a file called <code class="docutils literal notranslate"><span class="pre">prog.py</span></code>, it can be run at the command line and provides useful help messages:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python prog.py -h <span class="go">usage: prog.py [-h] [--sum] N [N ...]</span> <span class="go">Process some integers.</span> <span class="go">positional arguments:</span> <span class="go"> N an integer for the accumulator</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --sum sum the integers (default: find the max)</span> </pre></div> </div> <p>When run with the appropriate arguments, it prints either the sum or the max of the command-line integers:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python prog.py <span class="m">1</span> <span class="m">2</span> <span class="m">3</span> <span class="m">4</span> <span class="go">4</span> <span class="gp">$</span> python prog.py <span class="m">1</span> <span class="m">2</span> <span class="m">3</span> <span class="m">4</span> --sum <span class="go">10</span> </pre></div> </div> <p>If invalid arguments are passed in, it will issue an error:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python prog.py a b c <span class="go">usage: prog.py [-h] [--sum] N [N ...]</span> <span class="go">prog.py: error: argument N: invalid int value: 'a'</span> </pre></div> </div> <p>The following sections walk you through this example.</p> <div class="section" id="creating-a-parser"> <h3>16.4.1.1. Creating a parser<a class="headerlink" href="#creating-a-parser" title="Permalink to this headline">¶</a></h3> <p>The first step in using the <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> is creating an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> object:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">description</span><span class="o">=</span><span class="s1">'Process some integers.'</span><span class="p">)</span> </pre></div> </div> <p>The <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> object will hold all the information necessary to parse the command line into Python data types.</p> </div> <div class="section" id="adding-arguments"> <h3>16.4.1.2. Adding arguments<a class="headerlink" href="#adding-arguments" title="Permalink to this headline">¶</a></h3> <p>Filling an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> with information about program arguments is done by making calls to the <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> method. Generally, these calls tell the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> how to take the strings on the command line and turn them into objects. This information is stored and used when <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> is called. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'integers'</span><span class="p">,</span> <span class="n">metavar</span><span class="o">=</span><span class="s1">'N'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">help</span><span class="o">=</span><span class="s1">'an integer for the accumulator'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--sum'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'accumulate'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_const'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">const</span><span class="o">=</span><span class="nb">sum</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="nb">max</span><span class="p">,</span> <span class="gp">... </span> <span class="n">help</span><span class="o">=</span><span class="s1">'sum the integers (default: find the max)'</span><span class="p">)</span> </pre></div> </div> <p>Later, calling <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> will return an object with two attributes, <code class="docutils literal notranslate"><span class="pre">integers</span></code> and <code class="docutils literal notranslate"><span class="pre">accumulate</span></code>. The <code class="docutils literal notranslate"><span class="pre">integers</span></code> attribute will be a list of one or more ints, and the <code class="docutils literal notranslate"><span class="pre">accumulate</span></code> attribute will be either the <a class="reference internal" href="functions.html#sum" title="sum"><code class="xref py py-func docutils literal notranslate"><span class="pre">sum()</span></code></a> function, if <code class="docutils literal notranslate"><span class="pre">--sum</span></code> was specified at the command line, or the <a class="reference internal" href="functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a> function if it was not.</p> </div> <div class="section" id="parsing-arguments"> <h3>16.4.1.3. Parsing arguments<a class="headerlink" href="#parsing-arguments" title="Permalink to this headline">¶</a></h3> <p><a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> parses arguments through the <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> method. This will inspect the command line, convert each argument to the appropriate type and then invoke the appropriate action. In most cases, this means a simple <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a> object will be built up from attributes parsed out of the command line:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--sum'</span><span class="p">,</span> <span class="s1">'7'</span><span class="p">,</span> <span class="s1">'-1'</span><span class="p">,</span> <span class="s1">'42'</span><span class="p">])</span> <span class="go">Namespace(accumulate=<built-in function sum>, integers=[7, -1, 42])</span> </pre></div> </div> <p>In a script, <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> will typically be called with no arguments, and the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> will automatically determine the command-line arguments from <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a>.</p> </div> </div> <div class="section" id="argumentparser-objects"> <h2>16.4.2. ArgumentParser objects<a class="headerlink" href="#argumentparser-objects" title="Permalink to this headline">¶</a></h2> <dl class="class"> <dt id="argparse.ArgumentParser"> <em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">ArgumentParser</code><span class="sig-paren">(</span><em>prog=None</em>, <em>usage=None</em>, <em>description=None</em>, <em>epilog=None</em>, <em>parents=[]</em>, <em>formatter_class=argparse.HelpFormatter</em>, <em>prefix_chars='-'</em>, <em>fromfile_prefix_chars=None</em>, <em>argument_default=None</em>, <em>conflict_handler='error'</em>, <em>add_help=True</em>, <em>allow_abbrev=True</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser" title="Permalink to this definition">¶</a></dt> <dd><p>Create a new <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> object. All parameters should be passed as keyword arguments. Each parameter has its own more detailed description below, but in short they are:</p> <ul class="simple"> <li><a class="reference internal" href="#prog">prog</a> - The name of the program (default: <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code>)</li> <li><a class="reference internal" href="#usage">usage</a> - The string describing the program usage (default: generated from arguments added to parser)</li> <li><a class="reference internal" href="#description">description</a> - Text to display before the argument help (default: none)</li> <li><a class="reference internal" href="#epilog">epilog</a> - Text to display after the argument help (default: none)</li> <li><a class="reference internal" href="#parents">parents</a> - A list of <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects whose arguments should also be included</li> <li><a class="reference internal" href="#formatter-class">formatter_class</a> - A class for customizing the help output</li> <li><a class="reference internal" href="#prefix-chars">prefix_chars</a> - The set of characters that prefix optional arguments (default: ‘-‘)</li> <li><a class="reference internal" href="#fromfile-prefix-chars">fromfile_prefix_chars</a> - The set of characters that prefix files from which additional arguments should be read (default: <code class="docutils literal notranslate"><span class="pre">None</span></code>)</li> <li><a class="reference internal" href="#argument-default">argument_default</a> - The global default value for arguments (default: <code class="docutils literal notranslate"><span class="pre">None</span></code>)</li> <li><a class="reference internal" href="#conflict-handler">conflict_handler</a> - The strategy for resolving conflicting optionals (usually unnecessary)</li> <li><a class="reference internal" href="#add-help">add_help</a> - Add a <code class="docutils literal notranslate"><span class="pre">-h/--help</span></code> option to the parser (default: <code class="docutils literal notranslate"><span class="pre">True</span></code>)</li> <li><a class="reference internal" href="#allow-abbrev">allow_abbrev</a> - Allows long options to be abbreviated if the abbreviation is unambiguous. (default: <code class="docutils literal notranslate"><span class="pre">True</span></code>)</li> </ul> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.5: </span><em>allow_abbrev</em> parameter was added.</p> </div> </dd></dl> <p>The following sections describe how each of these are used.</p> <div class="section" id="prog"> <h3>16.4.2.1. prog<a class="headerlink" href="#prog" title="Permalink to this headline">¶</a></h3> <p>By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects use <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> to determine how to display the name of the program in help messages. This default is almost always desirable because it will make the help messages match how the program was invoked on the command line. For example, consider a file named <code class="docutils literal notranslate"><span class="pre">myprogram.py</span></code> with the following code:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> </pre></div> </div> <p>The help for this program will display <code class="docutils literal notranslate"><span class="pre">myprogram.py</span></code> as the program name (regardless of where the program was invoked from):</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python myprogram.py --help <span class="go">usage: myprogram.py [-h] [--foo FOO]</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo FOO foo help</span> <span class="gp">$</span> <span class="nb">cd</span> .. <span class="gp">$</span> python subdir/myprogram.py --help <span class="go">usage: myprogram.py [-h] [--foo FOO]</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo FOO foo help</span> </pre></div> </div> <p>To change this default behavior, another value can be supplied using the <code class="docutils literal notranslate"><span class="pre">prog=</span></code> argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'myprogram'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: myprogram [-h]</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> </pre></div> </div> <p>Note that the program name, whether determined from <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> or from the <code class="docutils literal notranslate"><span class="pre">prog=</span></code> argument, is available to help messages using the <code class="docutils literal notranslate"><span class="pre">%(prog)s</span></code> format specifier.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'myprogram'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo of the </span><span class="si">%(prog)s</span><span class="s1"> program'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: myprogram [-h] [--foo FOO]</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo FOO foo of the myprogram program</span> </pre></div> </div> </div> <div class="section" id="usage"> <h3>16.4.2.2. usage<a class="headerlink" href="#usage" title="Permalink to this headline">¶</a></h3> <p>By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> calculates the usage message from the arguments it contains:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'bar help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [-h] [--foo [FOO]] bar [bar ...]</span> <span class="go">positional arguments:</span> <span class="go"> bar bar help</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo [FOO] foo help</span> </pre></div> </div> <p>The default message can be overridden with the <code class="docutils literal notranslate"><span class="pre">usage=</span></code> keyword argument:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">usage</span><span class="o">=</span><span class="s1">'</span><span class="si">%(prog)s</span><span class="s1"> [options]'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'bar help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [options]</span> <span class="go">positional arguments:</span> <span class="go"> bar bar help</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo [FOO] foo help</span> </pre></div> </div> <p>The <code class="docutils literal notranslate"><span class="pre">%(prog)s</span></code> format specifier is available to fill in the program name in your usage messages.</p> </div> <div class="section" id="description"> <h3>16.4.2.3. description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h3> <p>Most calls to the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> constructor will use the <code class="docutils literal notranslate"><span class="pre">description=</span></code> keyword argument. This argument gives a brief description of what the program does and how it works. In help messages, the description is displayed between the command-line usage string and the help messages for the various arguments:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">description</span><span class="o">=</span><span class="s1">'A foo that bars'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: argparse.py [-h]</span> <span class="go">A foo that bars</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> </pre></div> </div> <p>By default, the description will be line-wrapped so that it fits within the given space. To change this behavior, see the <a class="reference internal" href="#formatter-class">formatter_class</a> argument.</p> </div> <div class="section" id="epilog"> <h3>16.4.2.4. epilog<a class="headerlink" href="#epilog" title="Permalink to this headline">¶</a></h3> <p>Some programs like to display additional description of the program after the description of the arguments. Such text can be specified using the <code class="docutils literal notranslate"><span class="pre">epilog=</span></code> argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span> <span class="gp">... </span> <span class="n">description</span><span class="o">=</span><span class="s1">'A foo that bars'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">epilog</span><span class="o">=</span><span class="s2">"And that's how you'd foo a bar"</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: argparse.py [-h]</span> <span class="go">A foo that bars</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go">And that's how you'd foo a bar</span> </pre></div> </div> <p>As with the <a class="reference internal" href="#description">description</a> argument, the <code class="docutils literal notranslate"><span class="pre">epilog=</span></code> text is by default line-wrapped, but this behavior can be adjusted with the <a class="reference internal" href="#formatter-class">formatter_class</a> argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>.</p> </div> <div class="section" id="parents"> <h3>16.4.2.5. parents<a class="headerlink" href="#parents" title="Permalink to this headline">¶</a></h3> <p>Sometimes, several parsers share a common set of arguments. Rather than repeating the definitions of these arguments, a single parser with all the shared arguments and passed to <code class="docutils literal notranslate"><span class="pre">parents=</span></code> argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> can be used. The <code class="docutils literal notranslate"><span class="pre">parents=</span></code> argument takes a list of <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects, collects all the positional and optional actions from them, and adds these actions to the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> object being constructed:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parent_parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">add_help</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parent_parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--parent'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">foo_parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">parents</span><span class="o">=</span><span class="p">[</span><span class="n">parent_parser</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">foo_parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">foo_parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--parent'</span><span class="p">,</span> <span class="s1">'2'</span><span class="p">,</span> <span class="s1">'XXX'</span><span class="p">])</span> <span class="go">Namespace(foo='XXX', parent=2)</span> <span class="gp">>>> </span><span class="n">bar_parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">parents</span><span class="o">=</span><span class="p">[</span><span class="n">parent_parser</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">bar_parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">bar_parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--bar'</span><span class="p">,</span> <span class="s1">'YYY'</span><span class="p">])</span> <span class="go">Namespace(bar='YYY', parent=None)</span> </pre></div> </div> <p>Note that most parent parsers will specify <code class="docutils literal notranslate"><span class="pre">add_help=False</span></code>. Otherwise, the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> will see two <code class="docutils literal notranslate"><span class="pre">-h/--help</span></code> options (one in the parent and one in the child) and raise an error.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">You must fully initialize the parsers before passing them via <code class="docutils literal notranslate"><span class="pre">parents=</span></code>. If you change the parent parsers after the child parser, those changes will not be reflected in the child.</p> </div> </div> <div class="section" id="formatter-class"> <h3>16.4.2.6. formatter_class<a class="headerlink" href="#formatter-class" title="Permalink to this headline">¶</a></h3> <p><a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects allow the help formatting to be customized by specifying an alternate formatting class. Currently, there are four such classes:</p> <dl class="class"> <dt id="argparse.RawDescriptionHelpFormatter"> <em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">RawDescriptionHelpFormatter</code><a class="headerlink" href="#argparse.RawDescriptionHelpFormatter" title="Permalink to this definition">¶</a></dt> <dt id="argparse.RawTextHelpFormatter"> <em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">RawTextHelpFormatter</code><a class="headerlink" href="#argparse.RawTextHelpFormatter" title="Permalink to this definition">¶</a></dt> <dt id="argparse.ArgumentDefaultsHelpFormatter"> <em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">ArgumentDefaultsHelpFormatter</code><a class="headerlink" href="#argparse.ArgumentDefaultsHelpFormatter" title="Permalink to this definition">¶</a></dt> <dt id="argparse.MetavarTypeHelpFormatter"> <em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">MetavarTypeHelpFormatter</code><a class="headerlink" href="#argparse.MetavarTypeHelpFormatter" title="Permalink to this definition">¶</a></dt> <dd></dd></dl> <p><a class="reference internal" href="#argparse.RawDescriptionHelpFormatter" title="argparse.RawDescriptionHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">RawDescriptionHelpFormatter</span></code></a> and <a class="reference internal" href="#argparse.RawTextHelpFormatter" title="argparse.RawTextHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">RawTextHelpFormatter</span></code></a> give more control over how textual descriptions are displayed. By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects line-wrap the <a class="reference internal" href="#description">description</a> and <a class="reference internal" href="#epilog">epilog</a> texts in command-line help messages:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span> <span class="gp">... </span> <span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">description</span><span class="o">=</span><span class="s1">'''this description</span> <span class="gp">... </span><span class="s1"> was indented weird</span> <span class="gp">... </span><span class="s1"> but that is okay'''</span><span class="p">,</span> <span class="gp">... </span> <span class="n">epilog</span><span class="o">=</span><span class="s1">'''</span> <span class="gp">... </span><span class="s1"> likewise for this epilog whose whitespace will</span> <span class="gp">... </span><span class="s1"> be cleaned up and whose words will be wrapped</span> <span class="gp">... </span><span class="s1"> across a couple lines'''</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [-h]</span> <span class="go">this description was indented weird but that is okay</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go">likewise for this epilog whose whitespace will be cleaned up and whose words</span> <span class="go">will be wrapped across a couple lines</span> </pre></div> </div> <p>Passing <a class="reference internal" href="#argparse.RawDescriptionHelpFormatter" title="argparse.RawDescriptionHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">RawDescriptionHelpFormatter</span></code></a> as <code class="docutils literal notranslate"><span class="pre">formatter_class=</span></code> indicates that <a class="reference internal" href="#description">description</a> and <a class="reference internal" href="#epilog">epilog</a> are already correctly formatted and should not be line-wrapped:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span> <span class="gp">... </span> <span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">formatter_class</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">RawDescriptionHelpFormatter</span><span class="p">,</span> <span class="gp">... </span> <span class="n">description</span><span class="o">=</span><span class="n">textwrap</span><span class="o">.</span><span class="n">dedent</span><span class="p">(</span><span class="s1">'''</span><span class="se">\</span> <span class="gp">... </span><span class="s1"> Please do not mess up this text!</span> <span class="gp">... </span><span class="s1"> --------------------------------</span> <span class="gp">... </span><span class="s1"> I have indented it</span> <span class="gp">... </span><span class="s1"> exactly the way</span> <span class="gp">... </span><span class="s1"> I want it</span> <span class="gp">... </span><span class="s1"> '''</span><span class="p">))</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [-h]</span> <span class="go">Please do not mess up this text!</span> <span class="go">--------------------------------</span> <span class="go"> I have indented it</span> <span class="go"> exactly the way</span> <span class="go"> I want it</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> </pre></div> </div> <p><a class="reference internal" href="#argparse.RawTextHelpFormatter" title="argparse.RawTextHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">RawTextHelpFormatter</span></code></a> maintains whitespace for all sorts of help text, including argument descriptions. However, multiple new lines are replaced with one. If you wish to preserve multiple blank lines, add spaces between the newlines.</p> <p><a class="reference internal" href="#argparse.ArgumentDefaultsHelpFormatter" title="argparse.ArgumentDefaultsHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentDefaultsHelpFormatter</span></code></a> automatically adds information about default values to each of the argument help messages:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span> <span class="gp">... </span> <span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">formatter_class</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentDefaultsHelpFormatter</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">42</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'FOO!'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'*'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span> <span class="n">help</span><span class="o">=</span><span class="s1">'BAR!'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [-h] [--foo FOO] [bar [bar ...]]</span> <span class="go">positional arguments:</span> <span class="go"> bar BAR! (default: [1, 2, 3])</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo FOO FOO! (default: 42)</span> </pre></div> </div> <p><a class="reference internal" href="#argparse.MetavarTypeHelpFormatter" title="argparse.MetavarTypeHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">MetavarTypeHelpFormatter</span></code></a> uses the name of the <a class="reference internal" href="#type">type</a> argument for each argument as the display name for its values (rather than using the <a class="reference internal" href="#dest">dest</a> as the regular formatter does):</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span> <span class="gp">... </span> <span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">formatter_class</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">MetavarTypeHelpFormatter</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">float</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [-h] [--foo int] float</span> <span class="go">positional arguments:</span> <span class="go"> float</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo int</span> </pre></div> </div> </div> <div class="section" id="prefix-chars"> <h3>16.4.2.7. prefix_chars<a class="headerlink" href="#prefix-chars" title="Permalink to this headline">¶</a></h3> <p>Most command-line options will use <code class="docutils literal notranslate"><span class="pre">-</span></code> as the prefix, e.g. <code class="docutils literal notranslate"><span class="pre">-f/--foo</span></code>. Parsers that need to support different or additional prefix characters, e.g. for options like <code class="docutils literal notranslate"><span class="pre">+f</span></code> or <code class="docutils literal notranslate"><span class="pre">/foo</span></code>, may specify them using the <code class="docutils literal notranslate"><span class="pre">prefix_chars=</span></code> argument to the ArgumentParser constructor:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">prefix_chars</span><span class="o">=</span><span class="s1">'-+'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'+f'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'++bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'+f X ++bar Y'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bar='Y', f='X')</span> </pre></div> </div> <p>The <code class="docutils literal notranslate"><span class="pre">prefix_chars=</span></code> argument defaults to <code class="docutils literal notranslate"><span class="pre">'-'</span></code>. Supplying a set of characters that does not include <code class="docutils literal notranslate"><span class="pre">-</span></code> will cause <code class="docutils literal notranslate"><span class="pre">-f/--foo</span></code> options to be disallowed.</p> </div> <div class="section" id="fromfile-prefix-chars"> <h3>16.4.2.8. fromfile_prefix_chars<a class="headerlink" href="#fromfile-prefix-chars" title="Permalink to this headline">¶</a></h3> <p>Sometimes, for example when dealing with a particularly long argument lists, it may make sense to keep the list of arguments in a file rather than typing it out at the command line. If the <code class="docutils literal notranslate"><span class="pre">fromfile_prefix_chars=</span></code> argument is given to the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> constructor, then arguments that start with any of the specified characters will be treated as files, and will be replaced by the arguments they contain. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'args.txt'</span><span class="p">,</span> <span class="s1">'w'</span><span class="p">)</span> <span class="k">as</span> <span class="n">fp</span><span class="p">:</span> <span class="gp">... </span> <span class="n">fp</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">'-f</span><span class="se">\n</span><span class="s1">bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">fromfile_prefix_chars</span><span class="o">=</span><span class="s1">'@'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'foo'</span><span class="p">,</span> <span class="s1">'@args.txt'</span><span class="p">])</span> <span class="go">Namespace(f='bar')</span> </pre></div> </div> <p>Arguments read from a file must by default be one per line (but see also <a class="reference internal" href="#argparse.ArgumentParser.convert_arg_line_to_args" title="argparse.ArgumentParser.convert_arg_line_to_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">convert_arg_line_to_args()</span></code></a>) and are treated as if they were in the same place as the original file referencing argument on the command line. So in the example above, the expression <code class="docutils literal notranslate"><span class="pre">['-f',</span> <span class="pre">'foo',</span> <span class="pre">'@args.txt']</span></code> is considered equivalent to the expression <code class="docutils literal notranslate"><span class="pre">['-f',</span> <span class="pre">'foo',</span> <span class="pre">'-f',</span> <span class="pre">'bar']</span></code>.</p> <p>The <code class="docutils literal notranslate"><span class="pre">fromfile_prefix_chars=</span></code> argument defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>, meaning that arguments will never be treated as file references.</p> </div> <div class="section" id="argument-default"> <h3>16.4.2.9. argument_default<a class="headerlink" href="#argument-default" title="Permalink to this headline">¶</a></h3> <p>Generally, argument defaults are specified either by passing a default to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> or by calling the <a class="reference internal" href="#argparse.ArgumentParser.set_defaults" title="argparse.ArgumentParser.set_defaults"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set_defaults()</span></code></a> methods with a specific set of name-value pairs. Sometimes however, it may be useful to specify a single parser-wide default for arguments. This can be accomplished by passing the <code class="docutils literal notranslate"><span class="pre">argument_default=</span></code> keyword argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>. For example, to globally suppress attribute creation on <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> calls, we supply <code class="docutils literal notranslate"><span class="pre">argument_default=SUPPRESS</span></code>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">argument_default</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">SUPPRESS</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'1'</span><span class="p">,</span> <span class="s1">'BAR'</span><span class="p">])</span> <span class="go">Namespace(bar='BAR', foo='1')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">Namespace()</span> </pre></div> </div> </div> <div class="section" id="allow-abbrev"> <span id="id1"></span><h3>16.4.2.10. allow_abbrev<a class="headerlink" href="#allow-abbrev" title="Permalink to this headline">¶</a></h3> <p>Normally, when you pass an argument list to the <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> method of an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>, it <a class="reference internal" href="#prefix-matching"><span class="std std-ref">recognizes abbreviations</span></a> of long options.</p> <p>This feature can be disabled by setting <code class="docutils literal notranslate"><span class="pre">allow_abbrev</span></code> to <code class="docutils literal notranslate"><span class="pre">False</span></code>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">allow_abbrev</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foobar'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foonley'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_false'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foon'</span><span class="p">])</span> <span class="go">usage: PROG [-h] [--foobar] [--foonley]</span> <span class="go">PROG: error: unrecognized arguments: --foon</span> </pre></div> </div> <div class="versionadded"> <p><span class="versionmodified">New in version 3.5.</span></p> </div> </div> <div class="section" id="conflict-handler"> <h3>16.4.2.11. conflict_handler<a class="headerlink" href="#conflict-handler" title="Permalink to this headline">¶</a></h3> <p><a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects do not allow two actions with the same option string. By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects raise an exception if an attempt is made to create an argument with an option string that is already in use:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'old foo help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'new foo help'</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> <span class="gr"> ..</span> <span class="gr">ArgumentError</span>: <span class="n">argument --foo: conflicting option string(s): --foo</span> </pre></div> </div> <p>Sometimes (e.g. when using <a class="reference internal" href="#parents">parents</a>) it may be useful to simply override any older arguments with the same option string. To get this behavior, the value <code class="docutils literal notranslate"><span class="pre">'resolve'</span></code> can be supplied to the <code class="docutils literal notranslate"><span class="pre">conflict_handler=</span></code> argument of <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">conflict_handler</span><span class="o">=</span><span class="s1">'resolve'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'old foo help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'new foo help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [-h] [-f FOO] [--foo FOO]</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> -f FOO old foo help</span> <span class="go"> --foo FOO new foo help</span> </pre></div> </div> <p>Note that <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects only remove an action if all of its option strings are overridden. So, in the example above, the old <code class="docutils literal notranslate"><span class="pre">-f/--foo</span></code> action is retained as the <code class="docutils literal notranslate"><span class="pre">-f</span></code> action, because only the <code class="docutils literal notranslate"><span class="pre">--foo</span></code> option string was overridden.</p> </div> <div class="section" id="add-help"> <h3>16.4.2.12. add_help<a class="headerlink" href="#add-help" title="Permalink to this headline">¶</a></h3> <p>By default, ArgumentParser objects add an option which simply displays the parser’s help message. For example, consider a file named <code class="docutils literal notranslate"><span class="pre">myprogram.py</span></code> containing the following code:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> </pre></div> </div> <p>If <code class="docutils literal notranslate"><span class="pre">-h</span></code> or <code class="docutils literal notranslate"><span class="pre">--help</span></code> is supplied at the command line, the ArgumentParser help will be printed:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python myprogram.py --help <span class="go">usage: myprogram.py [-h] [--foo FOO]</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo FOO foo help</span> </pre></div> </div> <p>Occasionally, it may be useful to disable the addition of this help option. This can be achieved by passing <code class="docutils literal notranslate"><span class="pre">False</span></code> as the <code class="docutils literal notranslate"><span class="pre">add_help=</span></code> argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">add_help</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [--foo FOO]</span> <span class="go">optional arguments:</span> <span class="go"> --foo FOO foo help</span> </pre></div> </div> <p>The help option is typically <code class="docutils literal notranslate"><span class="pre">-h/--help</span></code>. The exception to this is if the <code class="docutils literal notranslate"><span class="pre">prefix_chars=</span></code> is specified and does not include <code class="docutils literal notranslate"><span class="pre">-</span></code>, in which case <code class="docutils literal notranslate"><span class="pre">-h</span></code> and <code class="docutils literal notranslate"><span class="pre">--help</span></code> are not valid options. In this case, the first character in <code class="docutils literal notranslate"><span class="pre">prefix_chars</span></code> is used to prefix the help options:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">prefix_chars</span><span class="o">=</span><span class="s1">'+/'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [+h]</span> <span class="go">optional arguments:</span> <span class="go"> +h, ++help show this help message and exit</span> </pre></div> </div> </div> </div> <div class="section" id="the-add-argument-method"> <h2>16.4.3. The add_argument() method<a class="headerlink" href="#the-add-argument-method" title="Permalink to this headline">¶</a></h2> <dl class="method"> <dt id="argparse.ArgumentParser.add_argument"> <code class="descclassname">ArgumentParser.</code><code class="descname">add_argument</code><span class="sig-paren">(</span><em>name or flags...</em><span class="optional">[</span>, <em>action</em><span class="optional">]</span><span class="optional">[</span>, <em>nargs</em><span class="optional">]</span><span class="optional">[</span>, <em>const</em><span class="optional">]</span><span class="optional">[</span>, <em>default</em><span class="optional">]</span><span class="optional">[</span>, <em>type</em><span class="optional">]</span><span class="optional">[</span>, <em>choices</em><span class="optional">]</span><span class="optional">[</span>, <em>required</em><span class="optional">]</span><span class="optional">[</span>, <em>help</em><span class="optional">]</span><span class="optional">[</span>, <em>metavar</em><span class="optional">]</span><span class="optional">[</span>, <em>dest</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.add_argument" title="Permalink to this definition">¶</a></dt> <dd><p>Define how a single command-line argument should be parsed. Each parameter has its own more detailed description below, but in short they are:</p> <ul class="simple"> <li><a class="reference internal" href="#name-or-flags">name or flags</a> - Either a name or a list of option strings, e.g. <code class="docutils literal notranslate"><span class="pre">foo</span></code> or <code class="docutils literal notranslate"><span class="pre">-f,</span> <span class="pre">--foo</span></code>.</li> <li><a class="reference internal" href="#action">action</a> - The basic type of action to be taken when this argument is encountered at the command line.</li> <li><a class="reference internal" href="#nargs">nargs</a> - The number of command-line arguments that should be consumed.</li> <li><a class="reference internal" href="#const">const</a> - A constant value required by some <a class="reference internal" href="#action">action</a> and <a class="reference internal" href="#nargs">nargs</a> selections.</li> <li><a class="reference internal" href="#default">default</a> - The value produced if the argument is absent from the command line.</li> <li><a class="reference internal" href="#type">type</a> - The type to which the command-line argument should be converted.</li> <li><a class="reference internal" href="#choices">choices</a> - A container of the allowable values for the argument.</li> <li><a class="reference internal" href="#required">required</a> - Whether or not the command-line option may be omitted (optionals only).</li> <li><a class="reference internal" href="#help">help</a> - A brief description of what the argument does.</li> <li><a class="reference internal" href="#metavar">metavar</a> - A name for the argument in usage messages.</li> <li><a class="reference internal" href="#dest">dest</a> - The name of the attribute to be added to the object returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>.</li> </ul> </dd></dl> <p>The following sections describe how each of these are used.</p> <div class="section" id="name-or-flags"> <h3>16.4.3.1. name or flags<a class="headerlink" href="#name-or-flags" title="Permalink to this headline">¶</a></h3> <p>The <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> method must know whether an optional argument, like <code class="docutils literal notranslate"><span class="pre">-f</span></code> or <code class="docutils literal notranslate"><span class="pre">--foo</span></code>, or a positional argument, like a list of filenames, is expected. The first arguments passed to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> must therefore be either a series of flags, or a simple argument name. For example, an optional argument could be created like:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">)</span> </pre></div> </div> <p>while a positional argument could be created like:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">)</span> </pre></div> </div> <p>When <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> is called, optional arguments will be identified by the <code class="docutils literal notranslate"><span class="pre">-</span></code> prefix, and the remaining arguments will be assumed to be positional:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'BAR'</span><span class="p">])</span> <span class="go">Namespace(bar='BAR', foo=None)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'BAR'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'FOO'</span><span class="p">])</span> <span class="go">Namespace(bar='BAR', foo='FOO')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'FOO'</span><span class="p">])</span> <span class="go">usage: PROG [-h] [-f FOO] bar</span> <span class="go">PROG: error: the following arguments are required: bar</span> </pre></div> </div> </div> <div class="section" id="action"> <h3>16.4.3.2. action<a class="headerlink" href="#action" title="Permalink to this headline">¶</a></h3> <p><a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects associate command-line arguments with actions. These actions can do just about anything with the command-line arguments associated with them, though most actions simply add an attribute to the object returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>. The <code class="docutils literal notranslate"><span class="pre">action</span></code> keyword argument specifies how the command-line arguments should be handled. The supplied actions are:</p> <ul> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'store'</span></code> - This just stores the argument’s value. This is the default action. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo 1'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(foo='1')</span> </pre></div> </div> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'store_const'</span></code> - This stores the value specified by the <a class="reference internal" href="#const">const</a> keyword argument. The <code class="docutils literal notranslate"><span class="pre">'store_const'</span></code> action is most commonly used with optional arguments that specify some sort of flag. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_const'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="mi">42</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">])</span> <span class="go">Namespace(foo=42)</span> </pre></div> </div> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'store_true'</span></code> and <code class="docutils literal notranslate"><span class="pre">'store_false'</span></code> - These are special cases of <code class="docutils literal notranslate"><span class="pre">'store_const'</span></code> used for storing the values <code class="docutils literal notranslate"><span class="pre">True</span></code> and <code class="docutils literal notranslate"><span class="pre">False</span></code> respectively. In addition, they create default values of <code class="docutils literal notranslate"><span class="pre">False</span></code> and <code class="docutils literal notranslate"><span class="pre">True</span></code> respectively. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--bar'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_false'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--baz'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_false'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo --bar'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(foo=True, bar=False, baz=True)</span> </pre></div> </div> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'append'</span></code> - This stores a list, and appends each argument value to the list. This is useful to allow an option to be specified multiple times. Example usage:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'append'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo 1 --foo 2'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(foo=['1', '2'])</span> </pre></div> </div> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'append_const'</span></code> - This stores a list, and appends the value specified by the <a class="reference internal" href="#const">const</a> keyword argument to the list. (Note that the <a class="reference internal" href="#const">const</a> keyword argument defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>.) The <code class="docutils literal notranslate"><span class="pre">'append_const'</span></code> action is typically useful when multiple arguments need to store constants to the same list. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--str'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'types'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'append_const'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="nb">str</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--int'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'types'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'append_const'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--str --int'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(types=[<class 'str'>, <class 'int'>])</span> </pre></div> </div> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'count'</span></code> - This counts the number of times a keyword argument occurs. For example, this is useful for increasing verbosity levels:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--verbose'</span><span class="p">,</span> <span class="s1">'-v'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'count'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-vvv'</span><span class="p">])</span> <span class="go">Namespace(verbose=3)</span> </pre></div> </div> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'help'</span></code> - This prints a complete help message for all the options in the current parser and then exits. By default a help action is automatically added to the parser. See <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> for details of how the output is created.</p> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'version'</span></code> - This expects a <code class="docutils literal notranslate"><span class="pre">version=</span></code> keyword argument in the <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> call, and prints version information and exits when invoked:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">argparse</span> <span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--version'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'version'</span><span class="p">,</span> <span class="n">version</span><span class="o">=</span><span class="s1">'</span><span class="si">%(prog)s</span><span class="s1"> 2.0'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--version'</span><span class="p">])</span> <span class="go">PROG 2.0</span> </pre></div> </div> </li> </ul> <p>You may also specify an arbitrary action by passing an Action subclass or other object that implements the same interface. The recommended way to do this is to extend <a class="reference internal" href="#argparse.Action" title="argparse.Action"><code class="xref py py-class docutils literal notranslate"><span class="pre">Action</span></code></a>, overriding the <code class="docutils literal notranslate"><span class="pre">__call__</span></code> method and optionally the <code class="docutils literal notranslate"><span class="pre">__init__</span></code> method.</p> <p>An example of a custom action:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">FooAction</span><span class="p">(</span><span class="n">argparse</span><span class="o">.</span><span class="n">Action</span><span class="p">):</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">option_strings</span><span class="p">,</span> <span class="n">dest</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span> <span class="gp">... </span> <span class="k">if</span> <span class="n">nargs</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span> <span class="gp">... </span> <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s2">"nargs not allowed"</span><span class="p">)</span> <span class="gp">... </span> <span class="nb">super</span><span class="p">(</span><span class="n">FooAction</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="n">option_strings</span><span class="p">,</span> <span class="n">dest</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">parser</span><span class="p">,</span> <span class="n">namespace</span><span class="p">,</span> <span class="n">values</span><span class="p">,</span> <span class="n">option_string</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">%r</span><span class="s1"> </span><span class="si">%r</span><span class="s1"> </span><span class="si">%r</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">namespace</span><span class="p">,</span> <span class="n">values</span><span class="p">,</span> <span class="n">option_string</span><span class="p">))</span> <span class="gp">... </span> <span class="nb">setattr</span><span class="p">(</span><span class="n">namespace</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">dest</span><span class="p">,</span> <span class="n">values</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="n">FooAction</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="n">FooAction</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'1 --foo 2'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bar=None, foo=None) '1' None</span> <span class="go">Namespace(bar='1', foo=None) '2' '--foo'</span> <span class="gp">>>> </span><span class="n">args</span> <span class="go">Namespace(bar='1', foo='2')</span> </pre></div> </div> <p>For more details, see <a class="reference internal" href="#argparse.Action" title="argparse.Action"><code class="xref py py-class docutils literal notranslate"><span class="pre">Action</span></code></a>.</p> </div> <div class="section" id="nargs"> <h3>16.4.3.3. nargs<a class="headerlink" href="#nargs" title="Permalink to this headline">¶</a></h3> <p>ArgumentParser objects usually associate a single command-line argument with a single action to be taken. The <code class="docutils literal notranslate"><span class="pre">nargs</span></code> keyword argument associates a different number of command-line arguments with a single action. The supported values are:</p> <ul> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">N</span></code> (an integer). <code class="docutils literal notranslate"><span class="pre">N</span></code> arguments from the command line will be gathered together into a list. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'c --foo a b'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bar=['c'], foo=['a', 'b'])</span> </pre></div> </div> <p>Note that <code class="docutils literal notranslate"><span class="pre">nargs=1</span></code> produces a list of one item. This is different from the default, in which the item is produced by itself.</p> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'?'</span></code>. One argument will be consumed from the command line if possible, and produced as a single item. If no command-line argument is present, the value from <a class="reference internal" href="#default">default</a> will be produced. Note that for optional arguments, there is an additional case - the option string is present but not followed by a command-line argument. In this case the value from <a class="reference internal" href="#const">const</a> will be produced. Some examples to illustrate this:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="s1">'c'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'d'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'d'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'XX'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'YY'</span><span class="p">])</span> <span class="go">Namespace(bar='XX', foo='YY')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'XX'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">])</span> <span class="go">Namespace(bar='XX', foo='c')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">Namespace(bar='d', foo='d')</span> </pre></div> </div> <p>One of the more common uses of <code class="docutils literal notranslate"><span class="pre">nargs='?'</span></code> is to allow optional input and output files:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'infile'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">FileType</span><span class="p">(</span><span class="s1">'r'</span><span class="p">),</span> <span class="gp">... </span> <span class="n">default</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stdin</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'outfile'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">FileType</span><span class="p">(</span><span class="s1">'w'</span><span class="p">),</span> <span class="gp">... </span> <span class="n">default</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'input.txt'</span><span class="p">,</span> <span class="s1">'output.txt'</span><span class="p">])</span> <span class="go">Namespace(infile=<_io.TextIOWrapper name='input.txt' encoding='UTF-8'>,</span> <span class="go"> outfile=<_io.TextIOWrapper name='output.txt' encoding='UTF-8'>)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>,</span> <span class="go"> outfile=<_io.TextIOWrapper name='<stdout>' encoding='UTF-8'>)</span> </pre></div> </div> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'*'</span></code>. All command-line arguments present are gathered into a list. Note that it generally doesn’t make much sense to have more than one positional argument with <code class="docutils literal notranslate"><span class="pre">nargs='*'</span></code>, but multiple optional arguments with <code class="docutils literal notranslate"><span class="pre">nargs='*'</span></code> is possible. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'*'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'*'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'baz'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'*'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'a b --foo x y --bar 1 2'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bar=['1', '2'], baz=['a', 'b'], foo=['x', 'y'])</span> </pre></div> </div> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">'+'</span></code>. Just like <code class="docutils literal notranslate"><span class="pre">'*'</span></code>, all command-line args present are gathered into a list. Additionally, an error message will be generated if there wasn’t at least one command-line argument present. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">])</span> <span class="go">Namespace(foo=['a', 'b'])</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">usage: PROG [-h] foo [foo ...]</span> <span class="go">PROG: error: the following arguments are required: foo</span> </pre></div> </div> </li> </ul> <ul id="argparse-remainder"> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">argparse.REMAINDER</span></code>. All the remaining command-line arguments are gathered into a list. This is commonly useful for command line utilities that dispatch to other command line utilities:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'command'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'args'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">REMAINDER</span><span class="p">)</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo B cmd --arg1 XX ZZ'</span><span class="o">.</span><span class="n">split</span><span class="p">()))</span> <span class="go">Namespace(args=['--arg1', 'XX', 'ZZ'], command='cmd', foo='B')</span> </pre></div> </div> </li> </ul> <p>If the <code class="docutils literal notranslate"><span class="pre">nargs</span></code> keyword argument is not provided, the number of arguments consumed is determined by the <a class="reference internal" href="#action">action</a>. Generally this means a single command-line argument will be consumed and a single item (not a list) will be produced.</p> </div> <div class="section" id="const"> <h3>16.4.3.4. const<a class="headerlink" href="#const" title="Permalink to this headline">¶</a></h3> <p>The <code class="docutils literal notranslate"><span class="pre">const</span></code> argument of <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> is used to hold constant values that are not read from the command line but are required for the various <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> actions. The two most common uses of it are:</p> <ul class="simple"> <li>When <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> is called with <code class="docutils literal notranslate"><span class="pre">action='store_const'</span></code> or <code class="docutils literal notranslate"><span class="pre">action='append_const'</span></code>. These actions add the <code class="docutils literal notranslate"><span class="pre">const</span></code> value to one of the attributes of the object returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>. See the <a class="reference internal" href="#action">action</a> description for examples.</li> <li>When <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> is called with option strings (like <code class="docutils literal notranslate"><span class="pre">-f</span></code> or <code class="docutils literal notranslate"><span class="pre">--foo</span></code>) and <code class="docutils literal notranslate"><span class="pre">nargs='?'</span></code>. This creates an optional argument that can be followed by zero or one command-line arguments. When parsing the command line, if the option string is encountered with no command-line argument following it, the value of <code class="docutils literal notranslate"><span class="pre">const</span></code> will be assumed instead. See the <a class="reference internal" href="#nargs">nargs</a> description for examples.</li> </ul> <p>With the <code class="docutils literal notranslate"><span class="pre">'store_const'</span></code> and <code class="docutils literal notranslate"><span class="pre">'append_const'</span></code> actions, the <code class="docutils literal notranslate"><span class="pre">const</span></code> keyword argument must be given. For other actions, it defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p> </div> <div class="section" id="default"> <h3>16.4.3.5. default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h3> <p>All optional arguments and some positional arguments may be omitted at the command line. The <code class="docutils literal notranslate"><span class="pre">default</span></code> keyword argument of <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a>, whose value defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>, specifies what value should be used if the command-line argument is not present. For optional arguments, the <code class="docutils literal notranslate"><span class="pre">default</span></code> value is used when the option string was not present at the command line:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">42</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'2'</span><span class="p">])</span> <span class="go">Namespace(foo='2')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">Namespace(foo=42)</span> </pre></div> </div> <p>If the <code class="docutils literal notranslate"><span class="pre">default</span></code> value is a string, the parser parses the value as if it were a command-line argument. In particular, the parser applies any <a class="reference internal" href="#type">type</a> conversion argument, if provided, before setting the attribute on the <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a> return value. Otherwise, the parser uses the value as is:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--length'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'10'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--width'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mf">10.5</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="go">Namespace(length=10, width=10.5)</span> </pre></div> </div> <p>For positional arguments with <a class="reference internal" href="#nargs">nargs</a> equal to <code class="docutils literal notranslate"><span class="pre">?</span></code> or <code class="docutils literal notranslate"><span class="pre">*</span></code>, the <code class="docutils literal notranslate"><span class="pre">default</span></code> value is used when no command-line argument was present:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">42</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'a'</span><span class="p">])</span> <span class="go">Namespace(foo='a')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">Namespace(foo=42)</span> </pre></div> </div> <p>Providing <code class="docutils literal notranslate"><span class="pre">default=argparse.SUPPRESS</span></code> causes no attribute to be added if the command-line argument was not present:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">SUPPRESS</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">Namespace()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'1'</span><span class="p">])</span> <span class="go">Namespace(foo='1')</span> </pre></div> </div> </div> <div class="section" id="type"> <h3>16.4.3.6. type<a class="headerlink" href="#type" title="Permalink to this headline">¶</a></h3> <p>By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects read command-line arguments in as simple strings. However, quite often the command-line string should instead be interpreted as another type, like a <a class="reference internal" href="functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a> or <a class="reference internal" href="functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>. The <code class="docutils literal notranslate"><span class="pre">type</span></code> keyword argument of <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> allows any necessary type-checking and type conversions to be performed. Common built-in types and functions can be used directly as the value of the <code class="docutils literal notranslate"><span class="pre">type</span></code> argument:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">open</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'2 temp.txt'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bar=<_io.TextIOWrapper name='temp.txt' encoding='UTF-8'>, foo=2)</span> </pre></div> </div> <p>See the section on the <a class="reference internal" href="#default">default</a> keyword argument for information on when the <code class="docutils literal notranslate"><span class="pre">type</span></code> argument is applied to default arguments.</p> <p>To ease the use of various types of files, the argparse module provides the factory FileType which takes the <code class="docutils literal notranslate"><span class="pre">mode=</span></code>, <code class="docutils literal notranslate"><span class="pre">bufsize=</span></code>, <code class="docutils literal notranslate"><span class="pre">encoding=</span></code> and <code class="docutils literal notranslate"><span class="pre">errors=</span></code> arguments of the <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> function. For example, <code class="docutils literal notranslate"><span class="pre">FileType('w')</span></code> can be used to create a writable file:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">FileType</span><span class="p">(</span><span class="s1">'w'</span><span class="p">))</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'out.txt'</span><span class="p">])</span> <span class="go">Namespace(bar=<_io.TextIOWrapper name='out.txt' encoding='UTF-8'>)</span> </pre></div> </div> <p><code class="docutils literal notranslate"><span class="pre">type=</span></code> can take any callable that takes a single string argument and returns the converted value:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">perfect_square</span><span class="p">(</span><span class="n">string</span><span class="p">):</span> <span class="gp">... </span> <span class="n">value</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">string</span><span class="p">)</span> <span class="gp">... </span> <span class="n">sqrt</span> <span class="o">=</span> <span class="n">math</span><span class="o">.</span><span class="n">sqrt</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="gp">... </span> <span class="k">if</span> <span class="n">sqrt</span> <span class="o">!=</span> <span class="nb">int</span><span class="p">(</span><span class="n">sqrt</span><span class="p">):</span> <span class="gp">... </span> <span class="n">msg</span> <span class="o">=</span> <span class="s2">"</span><span class="si">%r</span><span class="s2"> is not a perfect square"</span> <span class="o">%</span> <span class="n">string</span> <span class="gp">... </span> <span class="k">raise</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentTypeError</span><span class="p">(</span><span class="n">msg</span><span class="p">)</span> <span class="gp">... </span> <span class="k">return</span> <span class="n">value</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">perfect_square</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'9'</span><span class="p">])</span> <span class="go">Namespace(foo=9)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'7'</span><span class="p">])</span> <span class="go">usage: PROG [-h] foo</span> <span class="go">PROG: error: argument foo: '7' is not a perfect square</span> </pre></div> </div> <p>The <a class="reference internal" href="#choices">choices</a> keyword argument may be more convenient for type checkers that simply check against a range of values:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">choices</span><span class="o">=</span><span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">10</span><span class="p">))</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'7'</span><span class="p">])</span> <span class="go">Namespace(foo=7)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'11'</span><span class="p">])</span> <span class="go">usage: PROG [-h] {5,6,7,8,9}</span> <span class="go">PROG: error: argument foo: invalid choice: 11 (choose from 5, 6, 7, 8, 9)</span> </pre></div> </div> <p>See the <a class="reference internal" href="#choices">choices</a> section for more details.</p> </div> <div class="section" id="choices"> <h3>16.4.3.7. choices<a class="headerlink" href="#choices" title="Permalink to this headline">¶</a></h3> <p>Some command-line arguments should be selected from a restricted set of values. These can be handled by passing a container object as the <em>choices</em> keyword argument to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a>. When the command line is parsed, argument values will be checked, and an error message will be displayed if the argument was not one of the acceptable values:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'game.py'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'move'</span><span class="p">,</span> <span class="n">choices</span><span class="o">=</span><span class="p">[</span><span class="s1">'rock'</span><span class="p">,</span> <span class="s1">'paper'</span><span class="p">,</span> <span class="s1">'scissors'</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'rock'</span><span class="p">])</span> <span class="go">Namespace(move='rock')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'fire'</span><span class="p">])</span> <span class="go">usage: game.py [-h] {rock,paper,scissors}</span> <span class="go">game.py: error: argument move: invalid choice: 'fire' (choose from 'rock',</span> <span class="go">'paper', 'scissors')</span> </pre></div> </div> <p>Note that inclusion in the <em>choices</em> container is checked after any <a class="reference internal" href="#type">type</a> conversions have been performed, so the type of the objects in the <em>choices</em> container should match the <a class="reference internal" href="#type">type</a> specified:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'doors.py'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'door'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">choices</span><span class="o">=</span><span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">4</span><span class="p">))</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'3'</span><span class="p">]))</span> <span class="go">Namespace(door=3)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'4'</span><span class="p">])</span> <span class="go">usage: doors.py [-h] {1,2,3}</span> <span class="go">doors.py: error: argument door: invalid choice: 4 (choose from 1, 2, 3)</span> </pre></div> </div> <p>Any object that supports the <code class="docutils literal notranslate"><span class="pre">in</span></code> operator can be passed as the <em>choices</em> value, so <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a> objects, <a class="reference internal" href="stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate"><span class="pre">set</span></code></a> objects, custom containers, etc. are all supported.</p> </div> <div class="section" id="required"> <h3>16.4.3.8. required<a class="headerlink" href="#required" title="Permalink to this headline">¶</a></h3> <p>In general, the <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module assumes that flags like <code class="docutils literal notranslate"><span class="pre">-f</span></code> and <code class="docutils literal notranslate"><span class="pre">--bar</span></code> indicate <em>optional</em> arguments, which can always be omitted at the command line. To make an option <em>required</em>, <code class="docutils literal notranslate"><span class="pre">True</span></code> can be specified for the <code class="docutils literal notranslate"><span class="pre">required=</span></code> keyword argument to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">required</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'BAR'</span><span class="p">])</span> <span class="go">Namespace(foo='BAR')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">usage: argparse.py [-h] [--foo FOO]</span> <span class="go">argparse.py: error: option --foo is required</span> </pre></div> </div> <p>As the example shows, if an option is marked as <code class="docutils literal notranslate"><span class="pre">required</span></code>, <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> will report an error if that option is not present at the command line.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">Required options are generally considered bad form because users expect <em>options</em> to be <em>optional</em>, and thus they should be avoided when possible.</p> </div> </div> <div class="section" id="help"> <h3>16.4.3.9. help<a class="headerlink" href="#help" title="Permalink to this headline">¶</a></h3> <p>The <code class="docutils literal notranslate"><span class="pre">help</span></code> value is a string containing a brief description of the argument. When a user requests help (usually by using <code class="docutils literal notranslate"><span class="pre">-h</span></code> or <code class="docutils literal notranslate"><span class="pre">--help</span></code> at the command line), these <code class="docutils literal notranslate"><span class="pre">help</span></code> descriptions will be displayed with each argument:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'frobble'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo the bars before frobbling'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">help</span><span class="o">=</span><span class="s1">'one of the bars to be frobbled'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-h'</span><span class="p">])</span> <span class="go">usage: frobble [-h] [--foo] bar [bar ...]</span> <span class="go">positional arguments:</span> <span class="go"> bar one of the bars to be frobbled</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo foo the bars before frobbling</span> </pre></div> </div> <p>The <code class="docutils literal notranslate"><span class="pre">help</span></code> strings can include various format specifiers to avoid repetition of things like the program name or the argument <a class="reference internal" href="#default">default</a>. The available specifiers include the program name, <code class="docutils literal notranslate"><span class="pre">%(prog)s</span></code> and most keyword arguments to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a>, e.g. <code class="docutils literal notranslate"><span class="pre">%(default)s</span></code>, <code class="docutils literal notranslate"><span class="pre">%(type)s</span></code>, etc.:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'frobble'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">42</span><span class="p">,</span> <span class="gp">... </span> <span class="n">help</span><span class="o">=</span><span class="s1">'the bar to </span><span class="si">%(prog)s</span><span class="s1"> (default: </span><span class="si">%(default)s</span><span class="s1">)'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: frobble [-h] [bar]</span> <span class="go">positional arguments:</span> <span class="go"> bar the bar to frobble (default: 42)</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> </pre></div> </div> <p>As the help string supports %-formatting, if you want a literal <code class="docutils literal notranslate"><span class="pre">%</span></code> to appear in the help string, you must escape it as <code class="docutils literal notranslate"><span class="pre">%%</span></code>.</p> <p><a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> supports silencing the help entry for certain options, by setting the <code class="docutils literal notranslate"><span class="pre">help</span></code> value to <code class="docutils literal notranslate"><span class="pre">argparse.SUPPRESS</span></code>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'frobble'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">SUPPRESS</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: frobble [-h]</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> </pre></div> </div> </div> <div class="section" id="metavar"> <h3>16.4.3.10. metavar<a class="headerlink" href="#metavar" title="Permalink to this headline">¶</a></h3> <p>When <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> generates help messages, it needs some way to refer to each expected argument. By default, ArgumentParser objects use the <a class="reference internal" href="#dest">dest</a> value as the “name” of each object. By default, for positional argument actions, the <a class="reference internal" href="#dest">dest</a> value is used directly, and for optional argument actions, the <a class="reference internal" href="#dest">dest</a> value is uppercased. So, a single positional argument with <code class="docutils literal notranslate"><span class="pre">dest='bar'</span></code> will be referred to as <code class="docutils literal notranslate"><span class="pre">bar</span></code>. A single optional argument <code class="docutils literal notranslate"><span class="pre">--foo</span></code> that should be followed by a single command-line argument will be referred to as <code class="docutils literal notranslate"><span class="pre">FOO</span></code>. An example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'X --foo Y'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bar='X', foo='Y')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: [-h] [--foo FOO] bar</span> <span class="go">positional arguments:</span> <span class="go"> bar</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo FOO</span> </pre></div> </div> <p>An alternative name can be specified with <code class="docutils literal notranslate"><span class="pre">metavar</span></code>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">metavar</span><span class="o">=</span><span class="s1">'YYY'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">metavar</span><span class="o">=</span><span class="s1">'XXX'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'X --foo Y'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bar='X', foo='Y')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: [-h] [--foo YYY] XXX</span> <span class="go">positional arguments:</span> <span class="go"> XXX</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo YYY</span> </pre></div> </div> <p>Note that <code class="docutils literal notranslate"><span class="pre">metavar</span></code> only changes the <em>displayed</em> name - the name of the attribute on the <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> object is still determined by the <a class="reference internal" href="#dest">dest</a> value.</p> <p>Different values of <code class="docutils literal notranslate"><span class="pre">nargs</span></code> may cause the metavar to be used multiple times. Providing a tuple to <code class="docutils literal notranslate"><span class="pre">metavar</span></code> specifies a different display for each of the arguments:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-x'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">metavar</span><span class="o">=</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="s1">'baz'</span><span class="p">))</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [-h] [-x X X] [--foo bar baz]</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> -x X X</span> <span class="go"> --foo bar baz</span> </pre></div> </div> </div> <div class="section" id="dest"> <h3>16.4.3.11. dest<a class="headerlink" href="#dest" title="Permalink to this headline">¶</a></h3> <p>Most <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> actions add some value as an attribute of the object returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>. The name of this attribute is determined by the <code class="docutils literal notranslate"><span class="pre">dest</span></code> keyword argument of <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a>. For positional argument actions, <code class="docutils literal notranslate"><span class="pre">dest</span></code> is normally supplied as the first argument to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'XXX'</span><span class="p">])</span> <span class="go">Namespace(bar='XXX')</span> </pre></div> </div> <p>For optional argument actions, the value of <code class="docutils literal notranslate"><span class="pre">dest</span></code> is normally inferred from the option strings. <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> generates the value of <code class="docutils literal notranslate"><span class="pre">dest</span></code> by taking the first long option string and stripping away the initial <code class="docutils literal notranslate"><span class="pre">--</span></code> string. If no long option strings were supplied, <code class="docutils literal notranslate"><span class="pre">dest</span></code> will be derived from the first short option string by stripping the initial <code class="docutils literal notranslate"><span class="pre">-</span></code> character. Any internal <code class="docutils literal notranslate"><span class="pre">-</span></code> characters will be converted to <code class="docutils literal notranslate"><span class="pre">_</span></code> characters to make sure the string is a valid attribute name. The examples below illustrate this behavior:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'--foo-bar'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-x'</span><span class="p">,</span> <span class="s1">'-y'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'-f 1 -x 2'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(foo_bar='1', x='2')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo 1 -y 2'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(foo_bar='1', x='2')</span> </pre></div> </div> <p><code class="docutils literal notranslate"><span class="pre">dest</span></code> allows a custom attribute name to be provided:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo XXX'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bar='XXX')</span> </pre></div> </div> </div> <div class="section" id="action-classes"> <h3>16.4.3.12. Action classes<a class="headerlink" href="#action-classes" title="Permalink to this headline">¶</a></h3> <p>Action classes implement the Action API, a callable which returns a callable which processes arguments from the command-line. Any object which follows this API may be passed as the <code class="docutils literal notranslate"><span class="pre">action</span></code> parameter to <code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code>.</p> <dl class="class"> <dt id="argparse.Action"> <em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">Action</code><span class="sig-paren">(</span><em>option_strings</em>, <em>dest</em>, <em>nargs=None</em>, <em>const=None</em>, <em>default=None</em>, <em>type=None</em>, <em>choices=None</em>, <em>required=False</em>, <em>help=None</em>, <em>metavar=None</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.Action" title="Permalink to this definition">¶</a></dt> <dd></dd></dl> <p>Action objects are used by an ArgumentParser to represent the information needed to parse a single argument from one or more strings from the command line. The Action class must accept the two positional arguments plus any keyword arguments passed to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ArgumentParser.add_argument()</span></code></a> except for the <code class="docutils literal notranslate"><span class="pre">action</span></code> itself.</p> <p>Instances of Action (or return value of any callable to the <code class="docutils literal notranslate"><span class="pre">action</span></code> parameter) should have attributes “dest”, “option_strings”, “default”, “type”, “required”, “help”, etc. defined. The easiest way to ensure these attributes are defined is to call <code class="docutils literal notranslate"><span class="pre">Action.__init__</span></code>.</p> <p>Action instances should be callable, so subclasses must override the <code class="docutils literal notranslate"><span class="pre">__call__</span></code> method, which should accept four parameters:</p> <ul class="simple"> <li><code class="docutils literal notranslate"><span class="pre">parser</span></code> - The ArgumentParser object which contains this action.</li> <li><code class="docutils literal notranslate"><span class="pre">namespace</span></code> - The <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a> object that will be returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>. Most actions add an attribute to this object using <a class="reference internal" href="functions.html#setattr" title="setattr"><code class="xref py py-func docutils literal notranslate"><span class="pre">setattr()</span></code></a>.</li> <li><code class="docutils literal notranslate"><span class="pre">values</span></code> - The associated command-line arguments, with any type conversions applied. Type conversions are specified with the <a class="reference internal" href="#type">type</a> keyword argument to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a>.</li> <li><code class="docutils literal notranslate"><span class="pre">option_string</span></code> - The option string that was used to invoke this action. The <code class="docutils literal notranslate"><span class="pre">option_string</span></code> argument is optional, and will be absent if the action is associated with a positional argument.</li> </ul> <p>The <code class="docutils literal notranslate"><span class="pre">__call__</span></code> method may perform arbitrary actions, but will typically set attributes on the <code class="docutils literal notranslate"><span class="pre">namespace</span></code> based on <code class="docutils literal notranslate"><span class="pre">dest</span></code> and <code class="docutils literal notranslate"><span class="pre">values</span></code>.</p> </div> </div> <div class="section" id="the-parse-args-method"> <h2>16.4.4. The parse_args() method<a class="headerlink" href="#the-parse-args-method" title="Permalink to this headline">¶</a></h2> <dl class="method"> <dt id="argparse.ArgumentParser.parse_args"> <code class="descclassname">ArgumentParser.</code><code class="descname">parse_args</code><span class="sig-paren">(</span><em>args=None</em>, <em>namespace=None</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.parse_args" title="Permalink to this definition">¶</a></dt> <dd><p>Convert argument strings to objects and assign them as attributes of the namespace. Return the populated namespace.</p> <p>Previous calls to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> determine exactly what objects are created and how they are assigned. See the documentation for <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> for details.</p> <ul class="simple"> <li><a class="reference internal" href="#args">args</a> - List of strings to parse. The default is taken from <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a>.</li> <li><a class="reference internal" href="#namespace">namespace</a> - An object to take the attributes. The default is a new empty <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a> object.</li> </ul> </dd></dl> <div class="section" id="option-value-syntax"> <h3>16.4.4.1. Option value syntax<a class="headerlink" href="#option-value-syntax" title="Permalink to this headline">¶</a></h3> <p>The <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> method supports several ways of specifying the value of an option (if it takes one). In the simplest case, the option and its value are passed as two separate arguments:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-x'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-x'</span><span class="p">,</span> <span class="s1">'X'</span><span class="p">])</span> <span class="go">Namespace(foo=None, x='X')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'FOO'</span><span class="p">])</span> <span class="go">Namespace(foo='FOO', x=None)</span> </pre></div> </div> <p>For long options (options with names longer than a single character), the option and value can also be passed as a single command-line argument, using <code class="docutils literal notranslate"><span class="pre">=</span></code> to separate them:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo=FOO'</span><span class="p">])</span> <span class="go">Namespace(foo='FOO', x=None)</span> </pre></div> </div> <p>For short options (options only one character long), the option and its value can be concatenated:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-xX'</span><span class="p">])</span> <span class="go">Namespace(foo=None, x='X')</span> </pre></div> </div> <p>Several short options can be joined together, using only a single <code class="docutils literal notranslate"><span class="pre">-</span></code> prefix, as long as only the last option (or none of them) requires a value:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-x'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-y'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-z'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-xyzZ'</span><span class="p">])</span> <span class="go">Namespace(x=True, y=True, z='Z')</span> </pre></div> </div> </div> <div class="section" id="invalid-arguments"> <h3>16.4.4.2. Invalid arguments<a class="headerlink" href="#invalid-arguments" title="Permalink to this headline">¶</a></h3> <p>While parsing the command line, <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> checks for a variety of errors, including ambiguous options, invalid types, invalid options, wrong number of positional arguments, etc. When it encounters such an error, it exits and prints the error along with a usage message:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">)</span> <span class="gp">>>> </span><span class="c1"># invalid type</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'spam'</span><span class="p">])</span> <span class="go">usage: PROG [-h] [--foo FOO] [bar]</span> <span class="go">PROG: error: argument --foo: invalid int value: 'spam'</span> <span class="gp">>>> </span><span class="c1"># invalid option</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--bar'</span><span class="p">])</span> <span class="go">usage: PROG [-h] [--foo FOO] [bar]</span> <span class="go">PROG: error: no such option: --bar</span> <span class="gp">>>> </span><span class="c1"># wrong number of arguments</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'badger'</span><span class="p">])</span> <span class="go">usage: PROG [-h] [--foo FOO] [bar]</span> <span class="go">PROG: error: extra arguments found: badger</span> </pre></div> </div> </div> <div class="section" id="arguments-containing"> <h3>16.4.4.3. Arguments containing <code class="docutils literal notranslate"><span class="pre">-</span></code><a class="headerlink" href="#arguments-containing" title="Permalink to this headline">¶</a></h3> <p>The <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> method attempts to give errors whenever the user has clearly made a mistake, but some situations are inherently ambiguous. For example, the command-line argument <code class="docutils literal notranslate"><span class="pre">-1</span></code> could either be an attempt to specify an option or an attempt to provide a positional argument. The <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> method is cautious here: positional arguments may only begin with <code class="docutils literal notranslate"><span class="pre">-</span></code> if they look like negative numbers and there are no options in the parser that look like negative numbers:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-x'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">)</span> <span class="gp">>>> </span><span class="c1"># no negative number options, so -1 is a positional argument</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-x'</span><span class="p">,</span> <span class="s1">'-1'</span><span class="p">])</span> <span class="go">Namespace(foo=None, x='-1')</span> <span class="gp">>>> </span><span class="c1"># no negative number options, so -1 and -5 are positional arguments</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-x'</span><span class="p">,</span> <span class="s1">'-1'</span><span class="p">,</span> <span class="s1">'-5'</span><span class="p">])</span> <span class="go">Namespace(foo='-5', x='-1')</span> <span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-1'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'one'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">)</span> <span class="gp">>>> </span><span class="c1"># negative number options present, so -1 is an option</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-1'</span><span class="p">,</span> <span class="s1">'X'</span><span class="p">])</span> <span class="go">Namespace(foo=None, one='X')</span> <span class="gp">>>> </span><span class="c1"># negative number options present, so -2 is an option</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-2'</span><span class="p">])</span> <span class="go">usage: PROG [-h] [-1 ONE] [foo]</span> <span class="go">PROG: error: no such option: -2</span> <span class="gp">>>> </span><span class="c1"># negative number options present, so both -1s are options</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-1'</span><span class="p">,</span> <span class="s1">'-1'</span><span class="p">])</span> <span class="go">usage: PROG [-h] [-1 ONE] [foo]</span> <span class="go">PROG: error: argument -1: expected one argument</span> </pre></div> </div> <p>If you have positional arguments that must begin with <code class="docutils literal notranslate"><span class="pre">-</span></code> and don’t look like negative numbers, you can insert the pseudo-argument <code class="docutils literal notranslate"><span class="pre">'--'</span></code> which tells <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> that everything after that is a positional argument:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--'</span><span class="p">,</span> <span class="s1">'-f'</span><span class="p">])</span> <span class="go">Namespace(foo='-f', one=None)</span> </pre></div> </div> </div> <div class="section" id="argument-abbreviations-prefix-matching"> <span id="prefix-matching"></span><h3>16.4.4.4. Argument abbreviations (prefix matching)<a class="headerlink" href="#argument-abbreviations-prefix-matching" title="Permalink to this headline">¶</a></h3> <p>The <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> method <a class="reference internal" href="#allow-abbrev"><span class="std std-ref">by default</span></a> allows long options to be abbreviated to a prefix, if the abbreviation is unambiguous (the prefix matches a unique option):</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-bacon'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-badger'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'-bac MMM'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bacon='MMM', badger=None)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'-bad WOOD'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">Namespace(bacon=None, badger='WOOD')</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'-ba BA'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="go">usage: PROG [-h] [-bacon BACON] [-badger BADGER]</span> <span class="go">PROG: error: ambiguous option: -ba could match -badger, -bacon</span> </pre></div> </div> <p>An error is produced for arguments that could produce more than one options. This feature can be disabled by setting <a class="reference internal" href="#allow-abbrev"><span class="std std-ref">allow_abbrev</span></a> to <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p> </div> <div class="section" id="beyond-sys-argv"> <span id="args"></span><h3>16.4.4.5. Beyond <code class="docutils literal notranslate"><span class="pre">sys.argv</span></code><a class="headerlink" href="#beyond-sys-argv" title="Permalink to this headline">¶</a></h3> <p>Sometimes it may be useful to have an ArgumentParser parse arguments other than those of <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a>. This can be accomplished by passing a list of strings to <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>. This is useful for testing at the interactive prompt:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span> <span class="gp">... </span> <span class="s1">'integers'</span><span class="p">,</span> <span class="n">metavar</span><span class="o">=</span><span class="s1">'int'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">choices</span><span class="o">=</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="gp">... </span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'an integer in the range 0..9'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span> <span class="gp">... </span> <span class="s1">'--sum'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'accumulate'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_const'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="nb">sum</span><span class="p">,</span> <span class="gp">... </span> <span class="n">default</span><span class="o">=</span><span class="nb">max</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'sum the integers (default: find the max)'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'1'</span><span class="p">,</span> <span class="s1">'2'</span><span class="p">,</span> <span class="s1">'3'</span><span class="p">,</span> <span class="s1">'4'</span><span class="p">])</span> <span class="go">Namespace(accumulate=<built-in function max>, integers=[1, 2, 3, 4])</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'1'</span><span class="p">,</span> <span class="s1">'2'</span><span class="p">,</span> <span class="s1">'3'</span><span class="p">,</span> <span class="s1">'4'</span><span class="p">,</span> <span class="s1">'--sum'</span><span class="p">])</span> <span class="go">Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3, 4])</span> </pre></div> </div> </div> <div class="section" id="the-namespace-object"> <span id="namespace"></span><h3>16.4.4.6. The Namespace object<a class="headerlink" href="#the-namespace-object" title="Permalink to this headline">¶</a></h3> <dl class="class"> <dt id="argparse.Namespace"> <em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">Namespace</code><a class="headerlink" href="#argparse.Namespace" title="Permalink to this definition">¶</a></dt> <dd><p>Simple class used by default by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> to create an object holding attributes and return it.</p> </dd></dl> <p>This class is deliberately simple, just an <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a> subclass with a readable string representation. If you prefer to have dict-like view of the attributes, you can use the standard Python idiom, <a class="reference internal" href="functions.html#vars" title="vars"><code class="xref py py-func docutils literal notranslate"><span class="pre">vars()</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'BAR'</span><span class="p">])</span> <span class="gp">>>> </span><span class="nb">vars</span><span class="p">(</span><span class="n">args</span><span class="p">)</span> <span class="go">{'foo': 'BAR'}</span> </pre></div> </div> <p>It may also be useful to have an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> assign attributes to an already existing object, rather than a new <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a> object. This can be achieved by specifying the <code class="docutils literal notranslate"><span class="pre">namespace=</span></code> keyword argument:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="gp">... </span> <span class="k">pass</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="n">args</span><span class="o">=</span><span class="p">[</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'BAR'</span><span class="p">],</span> <span class="n">namespace</span><span class="o">=</span><span class="n">c</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="n">foo</span> <span class="go">'BAR'</span> </pre></div> </div> </div> </div> <div class="section" id="other-utilities"> <h2>16.4.5. Other utilities<a class="headerlink" href="#other-utilities" title="Permalink to this headline">¶</a></h2> <div class="section" id="sub-commands"> <h3>16.4.5.1. Sub-commands<a class="headerlink" href="#sub-commands" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="argparse.ArgumentParser.add_subparsers"> <code class="descclassname">ArgumentParser.</code><code class="descname">add_subparsers</code><span class="sig-paren">(</span><span class="optional">[</span><em>title</em><span class="optional">]</span><span class="optional">[</span>, <em>description</em><span class="optional">]</span><span class="optional">[</span>, <em>prog</em><span class="optional">]</span><span class="optional">[</span>, <em>parser_class</em><span class="optional">]</span><span class="optional">[</span>, <em>action</em><span class="optional">]</span><span class="optional">[</span>, <em>option_string</em><span class="optional">]</span><span class="optional">[</span>, <em>dest</em><span class="optional">]</span><span class="optional">[</span>, <em>help</em><span class="optional">]</span><span class="optional">[</span>, <em>metavar</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.add_subparsers" title="Permalink to this definition">¶</a></dt> <dd><p>Many programs split up their functionality into a number of sub-commands, for example, the <code class="docutils literal notranslate"><span class="pre">svn</span></code> program can invoke sub-commands like <code class="docutils literal notranslate"><span class="pre">svn</span> <span class="pre">checkout</span></code>, <code class="docutils literal notranslate"><span class="pre">svn</span> <span class="pre">update</span></code>, and <code class="docutils literal notranslate"><span class="pre">svn</span> <span class="pre">commit</span></code>. Splitting up functionality this way can be a particularly good idea when a program performs several different functions which require different kinds of command-line arguments. <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> supports the creation of such sub-commands with the <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_subparsers()</span></code></a> method. The <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_subparsers()</span></code></a> method is normally called with no arguments and returns a special action object. This object has a single method, <code class="xref py py-meth docutils literal notranslate"><span class="pre">add_parser()</span></code>, which takes a command name and any <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> constructor arguments, and returns an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> object that can be modified as usual.</p> <p>Description of parameters:</p> <ul class="simple"> <li>title - title for the sub-parser group in help output; by default “subcommands” if description is provided, otherwise uses title for positional arguments</li> <li>description - description for the sub-parser group in help output, by default <code class="docutils literal notranslate"><span class="pre">None</span></code></li> <li>prog - usage information that will be displayed with sub-command help, by default the name of the program and any positional arguments before the subparser argument</li> <li>parser_class - class which will be used to create sub-parser instances, by default the class of the current parser (e.g. ArgumentParser)</li> <li><a class="reference internal" href="#action">action</a> - the basic type of action to be taken when this argument is encountered at the command line</li> <li><a class="reference internal" href="#dest">dest</a> - name of the attribute under which sub-command name will be stored; by default <code class="docutils literal notranslate"><span class="pre">None</span></code> and no value is stored</li> <li><a class="reference internal" href="#help">help</a> - help for sub-parser group in help output, by default <code class="docutils literal notranslate"><span class="pre">None</span></code></li> <li><a class="reference internal" href="#metavar">metavar</a> - string presenting available sub-commands in help; by default it is <code class="docutils literal notranslate"><span class="pre">None</span></code> and presents sub-commands in form {cmd1, cmd2, ..}</li> </ul> <p>Some example usage:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="c1"># create the top-level parser</span> <span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">subparsers</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_subparsers</span><span class="p">(</span><span class="n">help</span><span class="o">=</span><span class="s1">'sub-command help'</span><span class="p">)</span> <span class="go">>>></span> <span class="gp">>>> </span><span class="c1"># create the parser for the "a" command</span> <span class="gp">>>> </span><span class="n">parser_a</span> <span class="o">=</span> <span class="n">subparsers</span><span class="o">.</span><span class="n">add_parser</span><span class="p">(</span><span class="s1">'a'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'a help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser_a</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'bar help'</span><span class="p">)</span> <span class="go">>>></span> <span class="gp">>>> </span><span class="c1"># create the parser for the "b" command</span> <span class="gp">>>> </span><span class="n">parser_b</span> <span class="o">=</span> <span class="n">subparsers</span><span class="o">.</span><span class="n">add_parser</span><span class="p">(</span><span class="s1">'b'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'b help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser_b</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--baz'</span><span class="p">,</span> <span class="n">choices</span><span class="o">=</span><span class="s1">'XYZ'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'baz help'</span><span class="p">)</span> <span class="go">>>></span> <span class="gp">>>> </span><span class="c1"># parse some argument lists</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'12'</span><span class="p">])</span> <span class="go">Namespace(bar=12, foo=False)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">,</span> <span class="s1">'--baz'</span><span class="p">,</span> <span class="s1">'Z'</span><span class="p">])</span> <span class="go">Namespace(baz='Z', foo=True)</span> </pre></div> </div> <p>Note that the object returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> will only contain attributes for the main parser and the subparser that was selected by the command line (and not any other subparsers). So in the example above, when the <code class="docutils literal notranslate"><span class="pre">a</span></code> command is specified, only the <code class="docutils literal notranslate"><span class="pre">foo</span></code> and <code class="docutils literal notranslate"><span class="pre">bar</span></code> attributes are present, and when the <code class="docutils literal notranslate"><span class="pre">b</span></code> command is specified, only the <code class="docutils literal notranslate"><span class="pre">foo</span></code> and <code class="docutils literal notranslate"><span class="pre">baz</span></code> attributes are present.</p> <p>Similarly, when a help message is requested from a subparser, only the help for that particular parser will be printed. The help message will not include parent parser or sibling parser messages. (A help message for each subparser command, however, can be given by supplying the <code class="docutils literal notranslate"><span class="pre">help=</span></code> argument to <code class="xref py py-meth docutils literal notranslate"><span class="pre">add_parser()</span></code> as above.)</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--help'</span><span class="p">])</span> <span class="go">usage: PROG [-h] [--foo] {a,b} ...</span> <span class="go">positional arguments:</span> <span class="go"> {a,b} sub-command help</span> <span class="go"> a a help</span> <span class="go"> b b help</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --foo foo help</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'--help'</span><span class="p">])</span> <span class="go">usage: PROG a [-h] bar</span> <span class="go">positional arguments:</span> <span class="go"> bar bar help</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'b'</span><span class="p">,</span> <span class="s1">'--help'</span><span class="p">])</span> <span class="go">usage: PROG b [-h] [--baz {X,Y,Z}]</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go"> --baz {X,Y,Z} baz help</span> </pre></div> </div> <p>The <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_subparsers()</span></code></a> method also supports <code class="docutils literal notranslate"><span class="pre">title</span></code> and <code class="docutils literal notranslate"><span class="pre">description</span></code> keyword arguments. When either is present, the subparser’s commands will appear in their own group in the help output. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">subparsers</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_subparsers</span><span class="p">(</span><span class="n">title</span><span class="o">=</span><span class="s1">'subcommands'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">description</span><span class="o">=</span><span class="s1">'valid subcommands'</span><span class="p">,</span> <span class="gp">... </span> <span class="n">help</span><span class="o">=</span><span class="s1">'additional help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">subparsers</span><span class="o">.</span><span class="n">add_parser</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">subparsers</span><span class="o">.</span><span class="n">add_parser</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-h'</span><span class="p">])</span> <span class="go">usage: [-h] {foo,bar} ...</span> <span class="go">optional arguments:</span> <span class="go"> -h, --help show this help message and exit</span> <span class="go">subcommands:</span> <span class="go"> valid subcommands</span> <span class="go"> {foo,bar} additional help</span> </pre></div> </div> <p>Furthermore, <code class="docutils literal notranslate"><span class="pre">add_parser</span></code> supports an additional <code class="docutils literal notranslate"><span class="pre">aliases</span></code> argument, which allows multiple strings to refer to the same subparser. This example, like <code class="docutils literal notranslate"><span class="pre">svn</span></code>, aliases <code class="docutils literal notranslate"><span class="pre">co</span></code> as a shorthand for <code class="docutils literal notranslate"><span class="pre">checkout</span></code>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">subparsers</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_subparsers</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">checkout</span> <span class="o">=</span> <span class="n">subparsers</span><span class="o">.</span><span class="n">add_parser</span><span class="p">(</span><span class="s1">'checkout'</span><span class="p">,</span> <span class="n">aliases</span><span class="o">=</span><span class="p">[</span><span class="s1">'co'</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">checkout</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'co'</span><span class="p">,</span> <span class="s1">'bar'</span><span class="p">])</span> <span class="go">Namespace(foo='bar')</span> </pre></div> </div> <p>One particularly effective way of handling sub-commands is to combine the use of the <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_subparsers()</span></code></a> method with calls to <a class="reference internal" href="#argparse.ArgumentParser.set_defaults" title="argparse.ArgumentParser.set_defaults"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set_defaults()</span></code></a> so that each subparser knows which Python function it should execute. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="c1"># sub-command functions</span> <span class="gp">>>> </span><span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">args</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">x</span> <span class="o">*</span> <span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="k">def</span> <span class="nf">bar</span><span class="p">(</span><span class="n">args</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'((</span><span class="si">%s</span><span class="s1">))'</span> <span class="o">%</span> <span class="n">args</span><span class="o">.</span><span class="n">z</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="c1"># create the top-level parser</span> <span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">subparsers</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_subparsers</span><span class="p">()</span> <span class="go">>>></span> <span class="gp">>>> </span><span class="c1"># create the parser for the "foo" command</span> <span class="gp">>>> </span><span class="n">parser_foo</span> <span class="o">=</span> <span class="n">subparsers</span><span class="o">.</span><span class="n">add_parser</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser_foo</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-x'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser_foo</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'y'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">float</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser_foo</span><span class="o">.</span><span class="n">set_defaults</span><span class="p">(</span><span class="n">func</span><span class="o">=</span><span class="n">foo</span><span class="p">)</span> <span class="go">>>></span> <span class="gp">>>> </span><span class="c1"># create the parser for the "bar" command</span> <span class="gp">>>> </span><span class="n">parser_bar</span> <span class="o">=</span> <span class="n">subparsers</span><span class="o">.</span><span class="n">add_parser</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser_bar</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'z'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser_bar</span><span class="o">.</span><span class="n">set_defaults</span><span class="p">(</span><span class="n">func</span><span class="o">=</span><span class="n">bar</span><span class="p">)</span> <span class="go">>>></span> <span class="gp">>>> </span><span class="c1"># parse the args and call whatever function was selected</span> <span class="gp">>>> </span><span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'foo 1 -x 2'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="gp">>>> </span><span class="n">args</span><span class="o">.</span><span class="n">func</span><span class="p">(</span><span class="n">args</span><span class="p">)</span> <span class="go">2.0</span> <span class="go">>>></span> <span class="gp">>>> </span><span class="c1"># parse the args and call whatever function was selected</span> <span class="gp">>>> </span><span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'bar XYZYX'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="gp">>>> </span><span class="n">args</span><span class="o">.</span><span class="n">func</span><span class="p">(</span><span class="n">args</span><span class="p">)</span> <span class="go">((XYZYX))</span> </pre></div> </div> <p>This way, you can let <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> do the job of calling the appropriate function after argument parsing is complete. Associating functions with actions like this is typically the easiest way to handle the different actions for each of your subparsers. However, if it is necessary to check the name of the subparser that was invoked, the <code class="docutils literal notranslate"><span class="pre">dest</span></code> keyword argument to the <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_subparsers()</span></code></a> call will work:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">subparsers</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_subparsers</span><span class="p">(</span><span class="n">dest</span><span class="o">=</span><span class="s1">'subparser_name'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">subparser1</span> <span class="o">=</span> <span class="n">subparsers</span><span class="o">.</span><span class="n">add_parser</span><span class="p">(</span><span class="s1">'1'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">subparser1</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-x'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">subparser2</span> <span class="o">=</span> <span class="n">subparsers</span><span class="o">.</span><span class="n">add_parser</span><span class="p">(</span><span class="s1">'2'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">subparser2</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'y'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'2'</span><span class="p">,</span> <span class="s1">'frobble'</span><span class="p">])</span> <span class="go">Namespace(subparser_name='2', y='frobble')</span> </pre></div> </div> </dd></dl> </div> <div class="section" id="filetype-objects"> <h3>16.4.5.2. FileType objects<a class="headerlink" href="#filetype-objects" title="Permalink to this headline">¶</a></h3> <dl class="class"> <dt id="argparse.FileType"> <em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">FileType</code><span class="sig-paren">(</span><em>mode='r'</em>, <em>bufsize=-1</em>, <em>encoding=None</em>, <em>errors=None</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.FileType" title="Permalink to this definition">¶</a></dt> <dd><p>The <a class="reference internal" href="#argparse.FileType" title="argparse.FileType"><code class="xref py py-class docutils literal notranslate"><span class="pre">FileType</span></code></a> factory creates objects that can be passed to the type argument of <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ArgumentParser.add_argument()</span></code></a>. Arguments that have <a class="reference internal" href="#argparse.FileType" title="argparse.FileType"><code class="xref py py-class docutils literal notranslate"><span class="pre">FileType</span></code></a> objects as their type will open command-line arguments as files with the requested modes, buffer sizes, encodings and error handling (see the <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> function for more details):</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--raw'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">FileType</span><span class="p">(</span><span class="s1">'wb'</span><span class="p">,</span> <span class="mi">0</span><span class="p">))</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'out'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">FileType</span><span class="p">(</span><span class="s1">'w'</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s1">'UTF-8'</span><span class="p">))</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--raw'</span><span class="p">,</span> <span class="s1">'raw.dat'</span><span class="p">,</span> <span class="s1">'file.txt'</span><span class="p">])</span> <span class="go">Namespace(out=<_io.TextIOWrapper name='file.txt' mode='w' encoding='UTF-8'>, raw=<_io.FileIO name='raw.dat' mode='wb'>)</span> </pre></div> </div> <p>FileType objects understand the pseudo-argument <code class="docutils literal notranslate"><span class="pre">'-'</span></code> and automatically convert this into <code class="docutils literal notranslate"><span class="pre">sys.stdin</span></code> for readable <a class="reference internal" href="#argparse.FileType" title="argparse.FileType"><code class="xref py py-class docutils literal notranslate"><span class="pre">FileType</span></code></a> objects and <code class="docutils literal notranslate"><span class="pre">sys.stdout</span></code> for writable <a class="reference internal" href="#argparse.FileType" title="argparse.FileType"><code class="xref py py-class docutils literal notranslate"><span class="pre">FileType</span></code></a> objects:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'infile'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">FileType</span><span class="p">(</span><span class="s1">'r'</span><span class="p">))</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-'</span><span class="p">])</span> <span class="go">Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>)</span> </pre></div> </div> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4: </span>The <em>encodings</em> and <em>errors</em> keyword arguments.</p> </div> </dd></dl> </div> <div class="section" id="argument-groups"> <h3>16.4.5.3. Argument groups<a class="headerlink" href="#argument-groups" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="argparse.ArgumentParser.add_argument_group"> <code class="descclassname">ArgumentParser.</code><code class="descname">add_argument_group</code><span class="sig-paren">(</span><em>title=None</em>, <em>description=None</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.add_argument_group" title="Permalink to this definition">¶</a></dt> <dd><p>By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> groups command-line arguments into “positional arguments” and “optional arguments” when displaying help messages. When there is a better conceptual grouping of arguments than this default one, appropriate groups can be created using the <a class="reference internal" href="#argparse.ArgumentParser.add_argument_group" title="argparse.ArgumentParser.add_argument_group"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument_group()</span></code></a> method:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">add_help</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument_group</span><span class="p">(</span><span class="s1">'group'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'bar help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [--foo FOO] bar</span> <span class="go">group:</span> <span class="go"> bar bar help</span> <span class="go"> --foo FOO foo help</span> </pre></div> </div> <p>The <a class="reference internal" href="#argparse.ArgumentParser.add_argument_group" title="argparse.ArgumentParser.add_argument_group"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument_group()</span></code></a> method returns an argument group object which has an <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> method just like a regular <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>. When an argument is added to the group, the parser treats it just like a normal argument, but displays the argument in a separate group for help messages. The <a class="reference internal" href="#argparse.ArgumentParser.add_argument_group" title="argparse.ArgumentParser.add_argument_group"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument_group()</span></code></a> method accepts <em>title</em> and <em>description</em> arguments which can be used to customize this display:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">add_help</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group1</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument_group</span><span class="p">(</span><span class="s1">'group1'</span><span class="p">,</span> <span class="s1">'group1 description'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group1</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group2</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument_group</span><span class="p">(</span><span class="s1">'group2'</span><span class="p">,</span> <span class="s1">'group2 description'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group2</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--bar'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'bar help'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span> <span class="go">usage: PROG [--bar BAR] foo</span> <span class="go">group1:</span> <span class="go"> group1 description</span> <span class="go"> foo foo help</span> <span class="go">group2:</span> <span class="go"> group2 description</span> <span class="go"> --bar BAR bar help</span> </pre></div> </div> <p>Note that any arguments not in your user-defined groups will end up back in the usual “positional arguments” and “optional arguments” sections.</p> </dd></dl> </div> <div class="section" id="mutual-exclusion"> <h3>16.4.5.4. Mutual exclusion<a class="headerlink" href="#mutual-exclusion" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="argparse.ArgumentParser.add_mutually_exclusive_group"> <code class="descclassname">ArgumentParser.</code><code class="descname">add_mutually_exclusive_group</code><span class="sig-paren">(</span><em>required=False</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.add_mutually_exclusive_group" title="Permalink to this definition">¶</a></dt> <dd><p>Create a mutually exclusive group. <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> will make sure that only one of the arguments in the mutually exclusive group was present on the command line:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_mutually_exclusive_group</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--bar'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_false'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">])</span> <span class="go">Namespace(bar=True, foo=True)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--bar'</span><span class="p">])</span> <span class="go">Namespace(bar=False, foo=False)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'--bar'</span><span class="p">])</span> <span class="go">usage: PROG [-h] [--foo | --bar]</span> <span class="go">PROG: error: argument --bar: not allowed with argument --foo</span> </pre></div> </div> <p>The <a class="reference internal" href="#argparse.ArgumentParser.add_mutually_exclusive_group" title="argparse.ArgumentParser.add_mutually_exclusive_group"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_mutually_exclusive_group()</span></code></a> method also accepts a <em>required</em> argument, to indicate that at least one of the mutually exclusive arguments is required:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_mutually_exclusive_group</span><span class="p">(</span><span class="n">required</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--bar'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_false'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">usage: PROG [-h] (--foo | --bar)</span> <span class="go">PROG: error: one of the arguments --foo --bar is required</span> </pre></div> </div> <p>Note that currently mutually exclusive argument groups do not support the <em>title</em> and <em>description</em> arguments of <a class="reference internal" href="#argparse.ArgumentParser.add_argument_group" title="argparse.ArgumentParser.add_argument_group"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument_group()</span></code></a>.</p> </dd></dl> </div> <div class="section" id="parser-defaults"> <h3>16.4.5.5. Parser defaults<a class="headerlink" href="#parser-defaults" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="argparse.ArgumentParser.set_defaults"> <code class="descclassname">ArgumentParser.</code><code class="descname">set_defaults</code><span class="sig-paren">(</span><em>**kwargs</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.set_defaults" title="Permalink to this definition">¶</a></dt> <dd><p>Most of the time, the attributes of the object returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> will be fully determined by inspecting the command-line arguments and the argument actions. <a class="reference internal" href="#argparse.ArgumentParser.set_defaults" title="argparse.ArgumentParser.set_defaults"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set_defaults()</span></code></a> allows some additional attributes that are determined without any inspection of the command line to be added:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">set_defaults</span><span class="p">(</span><span class="n">bar</span><span class="o">=</span><span class="mi">42</span><span class="p">,</span> <span class="n">baz</span><span class="o">=</span><span class="s1">'badger'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'736'</span><span class="p">])</span> <span class="go">Namespace(bar=42, baz='badger', foo=736)</span> </pre></div> </div> <p>Note that parser-level defaults always override argument-level defaults:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">set_defaults</span><span class="p">(</span><span class="n">foo</span><span class="o">=</span><span class="s1">'spam'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span> <span class="go">Namespace(foo='spam')</span> </pre></div> </div> <p>Parser-level defaults can be particularly useful when working with multiple parsers. See the <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_subparsers()</span></code></a> method for an example of this type.</p> </dd></dl> <dl class="method"> <dt id="argparse.ArgumentParser.get_default"> <code class="descclassname">ArgumentParser.</code><code class="descname">get_default</code><span class="sig-paren">(</span><em>dest</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.get_default" title="Permalink to this definition">¶</a></dt> <dd><p>Get the default value for a namespace attribute, as set by either <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> or by <a class="reference internal" href="#argparse.ArgumentParser.set_defaults" title="argparse.ArgumentParser.set_defaults"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set_defaults()</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'badger'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">get_default</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">)</span> <span class="go">'badger'</span> </pre></div> </div> </dd></dl> </div> <div class="section" id="printing-help"> <h3>16.4.5.6. Printing help<a class="headerlink" href="#printing-help" title="Permalink to this headline">¶</a></h3> <p>In most typical applications, <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> will take care of formatting and printing any usage or error messages. However, several formatting methods are available:</p> <dl class="method"> <dt id="argparse.ArgumentParser.print_usage"> <code class="descclassname">ArgumentParser.</code><code class="descname">print_usage</code><span class="sig-paren">(</span><em>file=None</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.print_usage" title="Permalink to this definition">¶</a></dt> <dd><p>Print a brief description of how the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> should be invoked on the command line. If <em>file</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code>, <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a> is assumed.</p> </dd></dl> <dl class="method"> <dt id="argparse.ArgumentParser.print_help"> <code class="descclassname">ArgumentParser.</code><code class="descname">print_help</code><span class="sig-paren">(</span><em>file=None</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.print_help" title="Permalink to this definition">¶</a></dt> <dd><p>Print a help message, including the program usage and information about the arguments registered with the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>. If <em>file</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code>, <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a> is assumed.</p> </dd></dl> <p>There are also variants of these methods that simply return a string instead of printing it:</p> <dl class="method"> <dt id="argparse.ArgumentParser.format_usage"> <code class="descclassname">ArgumentParser.</code><code class="descname">format_usage</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.format_usage" title="Permalink to this definition">¶</a></dt> <dd><p>Return a string containing a brief description of how the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> should be invoked on the command line.</p> </dd></dl> <dl class="method"> <dt id="argparse.ArgumentParser.format_help"> <code class="descclassname">ArgumentParser.</code><code class="descname">format_help</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.format_help" title="Permalink to this definition">¶</a></dt> <dd><p>Return a string containing a help message, including the program usage and information about the arguments registered with the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>.</p> </dd></dl> </div> <div class="section" id="partial-parsing"> <h3>16.4.5.7. Partial parsing<a class="headerlink" href="#partial-parsing" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="argparse.ArgumentParser.parse_known_args"> <code class="descclassname">ArgumentParser.</code><code class="descname">parse_known_args</code><span class="sig-paren">(</span><em>args=None</em>, <em>namespace=None</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.parse_known_args" title="Permalink to this definition">¶</a></dt> <dd></dd></dl> <p>Sometimes a script may only parse a few of the command-line arguments, passing the remaining arguments on to another script or program. In these cases, the <a class="reference internal" href="#argparse.ArgumentParser.parse_known_args" title="argparse.ArgumentParser.parse_known_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_known_args()</span></code></a> method can be useful. It works much like <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> except that it does not produce an error when extra arguments are present. Instead, it returns a two item tuple containing the populated namespace and the list of remaining argument strings.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_known_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'--badger'</span><span class="p">,</span> <span class="s1">'BAR'</span><span class="p">,</span> <span class="s1">'spam'</span><span class="p">])</span> <span class="go">(Namespace(bar='BAR', foo=True), ['--badger', 'spam'])</span> </pre></div> </div> <div class="admonition warning"> <p class="first admonition-title">Warning</p> <p class="last"><a class="reference internal" href="#prefix-matching"><span class="std std-ref">Prefix matching</span></a> rules apply to <code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_known_args()</span></code>. The parser may consume an option even if it’s just a prefix of one of its known options, instead of leaving it in the remaining arguments list.</p> </div> </div> <div class="section" id="customizing-file-parsing"> <h3>16.4.5.8. Customizing file parsing<a class="headerlink" href="#customizing-file-parsing" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="argparse.ArgumentParser.convert_arg_line_to_args"> <code class="descclassname">ArgumentParser.</code><code class="descname">convert_arg_line_to_args</code><span class="sig-paren">(</span><em>arg_line</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.convert_arg_line_to_args" title="Permalink to this definition">¶</a></dt> <dd><p>Arguments that are read from a file (see the <em>fromfile_prefix_chars</em> keyword argument to the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> constructor) are read one argument per line. <a class="reference internal" href="#argparse.ArgumentParser.convert_arg_line_to_args" title="argparse.ArgumentParser.convert_arg_line_to_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">convert_arg_line_to_args()</span></code></a> can be overridden for fancier reading.</p> <p>This method takes a single argument <em>arg_line</em> which is a string read from the argument file. It returns a list of arguments parsed from this string. The method is called once per line read from the argument file, in order.</p> <p>A useful override of this method is one that treats each space-separated word as an argument. The following example demonstrates how to do this:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MyArgumentParser</span><span class="p">(</span><span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">):</span> <span class="k">def</span> <span class="nf">convert_arg_line_to_args</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">arg_line</span><span class="p">):</span> <span class="k">return</span> <span class="n">arg_line</span><span class="o">.</span><span class="n">split</span><span class="p">()</span> </pre></div> </div> </dd></dl> </div> <div class="section" id="exiting-methods"> <h3>16.4.5.9. Exiting methods<a class="headerlink" href="#exiting-methods" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="argparse.ArgumentParser.exit"> <code class="descclassname">ArgumentParser.</code><code class="descname">exit</code><span class="sig-paren">(</span><em>status=0</em>, <em>message=None</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.exit" title="Permalink to this definition">¶</a></dt> <dd><p>This method terminates the program, exiting with the specified <em>status</em> and, if given, it prints a <em>message</em> before that.</p> </dd></dl> <dl class="method"> <dt id="argparse.ArgumentParser.error"> <code class="descclassname">ArgumentParser.</code><code class="descname">error</code><span class="sig-paren">(</span><em>message</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.error" title="Permalink to this definition">¶</a></dt> <dd><p>This method prints a usage message including the <em>message</em> to the standard error and terminates the program with a status code of 2.</p> </dd></dl> </div> </div> <div class="section" id="upgrading-optparse-code"> <span id="id2"></span><h2>16.4.6. Upgrading optparse code<a class="headerlink" href="#upgrading-optparse-code" title="Permalink to this headline">¶</a></h2> <p>Originally, the <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module had attempted to maintain compatibility with <a class="reference internal" href="optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></a>. However, <a class="reference internal" href="optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></a> was difficult to extend transparently, particularly with the changes required to support the new <code class="docutils literal notranslate"><span class="pre">nargs=</span></code> specifiers and better usage messages. When most everything in <a class="reference internal" href="optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></a> had either been copy-pasted over or monkey-patched, it no longer seemed practical to try to maintain the backwards compatibility.</p> <p>The <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module improves on the standard library <a class="reference internal" href="optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></a> module in a number of ways including:</p> <ul class="simple"> <li>Handling positional arguments.</li> <li>Supporting sub-commands.</li> <li>Allowing alternative option prefixes like <code class="docutils literal notranslate"><span class="pre">+</span></code> and <code class="docutils literal notranslate"><span class="pre">/</span></code>.</li> <li>Handling zero-or-more and one-or-more style arguments.</li> <li>Producing more informative usage messages.</li> <li>Providing a much simpler interface for custom <code class="docutils literal notranslate"><span class="pre">type</span></code> and <code class="docutils literal notranslate"><span class="pre">action</span></code>.</li> </ul> <p>A partial upgrade path from <a class="reference internal" href="optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></a> to <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a>:</p> <ul class="simple"> <li>Replace all <a class="reference internal" href="optparse.html#optparse.OptionParser.add_option" title="optparse.OptionParser.add_option"><code class="xref py py-meth docutils literal notranslate"><span class="pre">optparse.OptionParser.add_option()</span></code></a> calls with <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ArgumentParser.add_argument()</span></code></a> calls.</li> <li>Replace <code class="docutils literal notranslate"><span class="pre">(options,</span> <span class="pre">args)</span> <span class="pre">=</span> <span class="pre">parser.parse_args()</span></code> with <code class="docutils literal notranslate"><span class="pre">args</span> <span class="pre">=</span> <span class="pre">parser.parse_args()</span></code> and add additional <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ArgumentParser.add_argument()</span></code></a> calls for the positional arguments. Keep in mind that what was previously called <code class="docutils literal notranslate"><span class="pre">options</span></code>, now in the <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> context is called <code class="docutils literal notranslate"><span class="pre">args</span></code>.</li> <li>Replace <a class="reference internal" href="optparse.html#optparse.OptionParser.disable_interspersed_args" title="optparse.OptionParser.disable_interspersed_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">optparse.OptionParser.disable_interspersed_args()</span></code></a> by setting <code class="docutils literal notranslate"><span class="pre">nargs</span></code> of a positional argument to <a class="reference internal" href="#argparse-remainder">argparse.REMAINDER</a>, or use <a class="reference internal" href="#argparse.ArgumentParser.parse_known_args" title="argparse.ArgumentParser.parse_known_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_known_args()</span></code></a> to collect unparsed argument strings in a separate list.</li> <li>Replace callback actions and the <code class="docutils literal notranslate"><span class="pre">callback_*</span></code> keyword arguments with <code class="docutils literal notranslate"><span class="pre">type</span></code> or <code class="docutils literal notranslate"><span class="pre">action</span></code> arguments.</li> <li>Replace string names for <code class="docutils literal notranslate"><span class="pre">type</span></code> keyword arguments with the corresponding type objects (e.g. int, float, complex, etc).</li> <li>Replace <code class="xref py py-class docutils literal notranslate"><span class="pre">optparse.Values</span></code> with <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a> and <code class="xref py py-exc docutils literal notranslate"><span class="pre">optparse.OptionError</span></code> and <code class="xref py py-exc docutils literal notranslate"><span class="pre">optparse.OptionValueError</span></code> with <code class="xref py py-exc docutils literal notranslate"><span class="pre">ArgumentError</span></code>.</li> <li>Replace strings with implicit arguments such as <code class="docutils literal notranslate"><span class="pre">%default</span></code> or <code class="docutils literal notranslate"><span class="pre">%prog</span></code> with the standard Python syntax to use dictionaries to format strings, that is, <code class="docutils literal notranslate"><span class="pre">%(default)s</span></code> and <code class="docutils literal notranslate"><span class="pre">%(prog)s</span></code>.</li> <li>Replace the OptionParser constructor <code class="docutils literal notranslate"><span class="pre">version</span></code> argument with a call to <code class="docutils literal notranslate"><span class="pre">parser.add_argument('--version',</span> <span class="pre">action='version',</span> <span class="pre">version='<the</span> <span class="pre">version>')</span></code>.</li> </ul> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">16.4. <code class="docutils literal notranslate"><span class="pre">argparse</span></code> — Parser for command-line options, arguments and sub-commands</a><ul> <li><a class="reference internal" href="#example">16.4.1. Example</a><ul> <li><a class="reference internal" href="#creating-a-parser">16.4.1.1. Creating a parser</a></li> <li><a class="reference internal" href="#adding-arguments">16.4.1.2. Adding arguments</a></li> <li><a class="reference internal" href="#parsing-arguments">16.4.1.3. Parsing arguments</a></li> </ul> </li> <li><a class="reference internal" href="#argumentparser-objects">16.4.2. ArgumentParser objects</a><ul> <li><a class="reference internal" href="#prog">16.4.2.1. prog</a></li> <li><a class="reference internal" href="#usage">16.4.2.2. usage</a></li> <li><a class="reference internal" href="#description">16.4.2.3. description</a></li> <li><a class="reference internal" href="#epilog">16.4.2.4. epilog</a></li> <li><a class="reference internal" href="#parents">16.4.2.5. parents</a></li> <li><a class="reference internal" href="#formatter-class">16.4.2.6. formatter_class</a></li> <li><a class="reference internal" href="#prefix-chars">16.4.2.7. prefix_chars</a></li> <li><a class="reference internal" href="#fromfile-prefix-chars">16.4.2.8. fromfile_prefix_chars</a></li> <li><a class="reference internal" href="#argument-default">16.4.2.9. argument_default</a></li> <li><a class="reference internal" href="#allow-abbrev">16.4.2.10. allow_abbrev</a></li> <li><a class="reference internal" href="#conflict-handler">16.4.2.11. conflict_handler</a></li> <li><a class="reference internal" href="#add-help">16.4.2.12. add_help</a></li> </ul> </li> <li><a class="reference internal" href="#the-add-argument-method">16.4.3. The add_argument() method</a><ul> <li><a class="reference internal" href="#name-or-flags">16.4.3.1. name or flags</a></li> <li><a class="reference internal" href="#action">16.4.3.2. action</a></li> <li><a class="reference internal" href="#nargs">16.4.3.3. nargs</a></li> <li><a class="reference internal" href="#const">16.4.3.4. const</a></li> <li><a class="reference internal" href="#default">16.4.3.5. default</a></li> <li><a class="reference internal" href="#type">16.4.3.6. type</a></li> <li><a class="reference internal" href="#choices">16.4.3.7. choices</a></li> <li><a class="reference internal" href="#required">16.4.3.8. required</a></li> <li><a class="reference internal" href="#help">16.4.3.9. help</a></li> <li><a class="reference internal" href="#metavar">16.4.3.10. metavar</a></li> <li><a class="reference internal" href="#dest">16.4.3.11. dest</a></li> <li><a class="reference internal" href="#action-classes">16.4.3.12. Action classes</a></li> </ul> </li> <li><a class="reference internal" href="#the-parse-args-method">16.4.4. The parse_args() method</a><ul> <li><a class="reference internal" href="#option-value-syntax">16.4.4.1. Option value syntax</a></li> <li><a class="reference internal" href="#invalid-arguments">16.4.4.2. Invalid arguments</a></li> <li><a class="reference internal" href="#arguments-containing">16.4.4.3. Arguments containing <code class="docutils literal notranslate"><span class="pre">-</span></code></a></li> <li><a class="reference internal" href="#argument-abbreviations-prefix-matching">16.4.4.4. Argument abbreviations (prefix matching)</a></li> <li><a class="reference internal" href="#beyond-sys-argv">16.4.4.5. Beyond <code class="docutils literal notranslate"><span class="pre">sys.argv</span></code></a></li> <li><a class="reference internal" href="#the-namespace-object">16.4.4.6. The Namespace object</a></li> </ul> </li> <li><a class="reference internal" href="#other-utilities">16.4.5. Other utilities</a><ul> <li><a class="reference internal" href="#sub-commands">16.4.5.1. Sub-commands</a></li> <li><a class="reference internal" href="#filetype-objects">16.4.5.2. FileType objects</a></li> <li><a class="reference internal" href="#argument-groups">16.4.5.3. Argument groups</a></li> <li><a class="reference internal" href="#mutual-exclusion">16.4.5.4. Mutual exclusion</a></li> <li><a class="reference internal" href="#parser-defaults">16.4.5.5. Parser defaults</a></li> <li><a class="reference internal" href="#printing-help">16.4.5.6. Printing help</a></li> <li><a class="reference internal" href="#partial-parsing">16.4.5.7. Partial parsing</a></li> <li><a class="reference internal" href="#customizing-file-parsing">16.4.5.8. Customizing file parsing</a></li> <li><a class="reference internal" href="#exiting-methods">16.4.5.9. Exiting methods</a></li> </ul> </li> <li><a class="reference internal" href="#upgrading-optparse-code">16.4.6. Upgrading optparse code</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="time.html" title="previous chapter">16.3. <code class="docutils literal notranslate"><span class="pre">time</span></code> — Time access and conversions</a></p> <h4>Next topic</h4> <p class="topless"><a href="getopt.html" title="next chapter">16.5. <code class="docutils literal notranslate"><span class="pre">getopt</span></code> — C-style parser for command line options</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/3.6/Doc/library/argparse.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="getopt.html" title="16.5. getopt — C-style parser for command line options" >next</a> |</li> <li class="right" > <a href="time.html" title="16.3. time — Time access and conversions" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="allos.html" >16. 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